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Digitized  by  the  Internet  Archive 

in  2009  with  funding  from 

Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/geologyofcomstocOObeck 


i^DVEIlTISEM:E]SrT. 


The  publications  of  the  United  States  Geological  Survey  are  issued  in  accordance  with  the  statute 

approved  March  3,  1879,  which  declares  that — 

The  publications  of  the  Geological  Survey  shall  consist  of  the  annual  report  of  operations,  geo- 
logical and  economic  maps  illustrating  the  resources  and  classifications  of  the  lands,  and  reports  upon 
general  and  economic  geology  and  paleontology.  The  annual  report  of  operations  of  the  Geological 
Survey  shall  accompany  the  annual  report  of  the  Secretary  of  the  Interior.  All  special  memoirs  and 
reports  of  said  Survey  shall  be  issued  in  uniform  quarto  series  if  deemed  necessary  by  the  Director,  but 
otherwise  in  ordinary  octavos.  Three  thousand  copies  of  each  shall  be  published  for  scientific  exchanges 
and  for  sale  at  the  price  of  publication ;  aud  all  literary  and  cartographic  materials  received  in  exchange 
shall  be  the  property  of  the  United  States  and  form  a  part  of  tbe  Ubraiy  of  the  organization :  And  the 
money  resulting  from  the  sale  of  such  publications  shall  be  covered  into  the  Treasury  of  the  United 
States. 

ANNUAL  REPORTS. 

From  the  above  it  will  be  seen  that  only  the  Annual  Reports,  which  form  parts  of  the  reports  of 
the  Secretary  of  the  Interior  and  are  printed  as  executive  documents,  are  available  for  gratuitous  dis- 
tribution. A  number  of  these  are  furnished  the  Survey  for  its  exchange  list,  but  the  bulk  of  them  are 
supplied  directly,  through  the  document  rooms  of  Congress,  to  members  of  the  Senate  and  House. 
Except,  therefore,  in  those  cases  in  which  an  extra  number  is  supplied  to  this  office  by  special  resolution, 
application  must  be  made  to  members  of  Congress  for  the  Annual  Reports,  as  for  all  other  executive 
documents. 

Of  these  Annuals  there  have  been  already  published : 

I.  First  Annual  Report  to  the  Hon.  Carl  Schurz,  by  Clarence  King,  8°,  Washington,  1880,  79  pp., 
I  map. — A  preliminary  report  describing  plan  of  organization  and  publications. 

II.  Report  of  the  Director  of  the  United  States  Geological  Survey  for  1880-'81,  by  J.  W.  Powell, 
8*^,  Washington,  1^82,  Iv.,  588  pp.,  61  plates,  1  map. 

CONTENTS. 

Report  of  tbe  Director,  pp.  i-lv.,  plates  1-7. 

Administrative  Reports  by  Heads  of  Divisions,  pp.  1-46,  plates  8  and  9. 

The  Physical  Geology  of  the  Grand  Canon  District,  by  Capt.  C.  E.  Dutton,  pp.  47-166,  plates  10-36 

Contribution  to  the  History  of  Lake  Bonneville,  by  G.  K.  Gilbert,  pp.  167-200,  plates  37-43. 

Abstract  of  Report  on  the  Geology  and  Mining  Industry  of  Leadville,  Colorado,  by  S.  F.  Emmons, 

pp.  201-290,  plates  44  and  45. 
A  Summary  of  the  Geology  of  the  Comstock  Lode  and  the  Washoe  District,  by  George  F.  Beckfer, 

pp.  «91-330,  plates  46  and  47. 
Production  of  Precious  Metals  in  the  United  States,  by  Clarence  King,  pp.  331-401,  plates  48-53. 
A  New  Method  of  Measuring  Heights  by  means  of  the  Barometer,  by  G.  K.  Gilbert,  pp.  403-565, 

plates  54-61. 
Index,  pp.  567-588. 

The  Third  and  Fourth  Annual  Reports  are  now  in  press. 

MONOGRAPHS. 

The  Monographs  of  the  Survey  are  printed  for  the  Survey  alone,  and  can  be  distributed  by  it  only 
through  a  fair  exchange  for  books  needed  in  its  library,  or  through  the  sale  of  those  copies  over  and 
above  the  number  needed  for  such  exchange.     They  are  not  for  gratuitous  distribution. 

So  far  as  already  determined  upon,  the  list  of  these  monographs  is  as  follows: 

I.  The  Precious  Metals,  by  Clarence  King.     In  preparation. 

II.  Tertiary  History  of  the  Grand  Canon  District,  with  atlas,  by  Capt.  C.  E.  Dutton.     Published. 
m.  Geology  of  the  Comstock  Lode  and  Washoe  District,  with  atlas,  by  George  F.  Becker. 


ii  ADVERTISEMENT. 

Published. 

IV.  Comstook  Mining  and  Miners,  by  Eliot  Lord.    In  press. 

v.  Copper-bearing  Eocks  of  Lake  Superior,  by  Professor  R.  D.  Irving.    In  press. 

"VI.  Older  Mesozoic  Flora  of  Virginia,  by  Prof.  William  M.  Fontaine.     In  press. 

Geology  and  Mining  Industry  of  Leadville,  with  atlas,  by  S.  F.  Emmons.    In  preparation. 

Geology  of  the  Eureka  Mining  District,  Nevada,  with  atlas,  by  Arnold  Hague.     In  preparation. 

Coal  of  the  United  States,  by  Prof  E.  Pumpelly.    In  preparation. 

Iron  of  the  United  States,  by  Prof.  E.  Pumpelly.     In  preparation. 

Lesser  Metals  and  General  Mining  Eesouroes,  by  Prof.  E.  Pumpelly.    In  preparation. 

Lake  Bonneville,  by  G.  K.  Gilbert.     In  preparation. 

Dinocerata.    A  monograph  on  an  extinct  order  of  Ungulates,  by  Prof.  O.  C.  Marsh.     In  press. 

Sauropoda,  by  Prof  O.  C.  Marsh.    In  preparation. 

Stegosauria,  by  Prof.  O.  C.  Marsh.     In  preparation. 

Of  these  monographs,  numbers  II.  and  III.  are  now  published,  viz : 

II.  Tertiary  History  of  the  Grand  Canon  District,  with  atlas,  by  C.  E.  Duttou.  1882,  4°,  264 
pp.,  42  plates,  and  atlas  of  26  double  sheets  folio.    Price  |10.12. 

III.  Geology  of  the  Comstock  Lode  and  Washoe  District,  with  atlas,  by  George  F.  Becker.  1882, 
4°,  422  pp.,  7  plates,  and  atlas  of  21  sheets  folio.     Price  |11. 

Numbers  IV.,  V.,  and  VI.  are  in  press  and  will  appear  in  quick  succession.  The  others,  to  which 
numbers  are  not  assigned,  are  in  preparation. 

BULLETINS. 

The  Bulletins  of  the  Survey  will  contain  such  papers  relating  to  the  general  purpose  of  its  work 
as  do  not  come  properly  under  the  heads  of  Annual  Reports,  or  Monographs. 

Each  of  these  Bulletins  will  contain  but  one  paper  and  be  complete  in  itself.  They  will,  how- 
ever, be  numbered  in  a  continuous  series,  and  will  in  time  be  united  into  volumes  of  convenient  size. 
To  facilitate  this  each  Bulletin  will  have  two  paginations,  one  proper  to  itself  and  another  which 
belongs  to  it  as  part  of  the  volume. 

Of  this  series  of  Bulletins  No.  1  is  already  published,  viz  : 

1.  On  Hypersthene-Andesite  and  on  Triclinic  Pyroxene  in  Augitic  Rooks,  by  Whitman  Cross, 
with  a  Geological  Sketch  of  Buffalo  Peaks,  Colorado,  by  S.  F.  Emmons.  1883.  40  pp.,  ti°.  Price  10 
cents. 

Correspondence  relating  to  the  publications  of  the  Survey,  and  all  remittances,  should  be  addressed 

to  the 

Director  of  the  United  States  Gkological  Survey, 

Washington,  D.  C. 
Washington,  D.  C,  March  1,  1883. 


47th  Congress,  )  HOUSE  OF  EEPEESENTATIVBS. 

Is*  Session.       | 


(  Mis.  Doc. 

(     No.  52. 


DEPARTMENT   OF   THE   INTERIOR 


MONOGRAPHS 


United  States  Geological  Survey 


VOLUME    III 


■      -'^Ai'  2     1952 
Wilbur  Cros-  r  u 
University  ef  c      '^'^'^ 
y  °i  Connecticut 


WASHIISrGTON 

GOVERNMENT     PRINTING     OFFICE 
1882 


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UNITED  STATES  GEOLOGICAL  SURVEY 

CLAEENCE  KING  DIRECTOR 


GEOLOGY 


COMSTOCK  LODE  AND  THE  WASHOE  DISTRICT 


^WITH    ATLAS 


By  GEOiiaE    F.    BECKER 


WASHINGTON 

GOVERNMENT     PRINTING    OFFICE 

1882 


American  Museum  of  Natukal  History, 

New  York  City. 
Hon.  Clarknce  King,  Director: 

Sir:  In  compliance  with  your  instructions  of  March  6,  1880,  directing- 
nie  to  report  upon  the  Geology,  Mineralogy,  Chemistry,  and  Physics  of  the 
CoMSTOCK  Lode,  I  have  the  honor  to  transmit  the  accompanying  report' 

Although  several  reports  on  the  Comstock  Lode  have  appeared  during 
the  past  twenty  years,  the  great  extension  of  the  mine  workings  and  the 
advances  in  geological  science  made  it  probable  that  additional  information 
of  value  would  result  from  a  reexamination  of  this  famous  ore-deposit. 
Administrative  duties  unfortunately  prevented  you  from  undertaking  the 
study  of  the  lower  portions  of  the  Lode,  the  upper  part  of  which  you  have 
made  so  familiar  to  geologists.  Under  these  circumstances,  you  did  me  the 
honor  to  select  me  as  your  substitute;  yet  you  did  not  abandon  all  share  in 
the  investigation,  since  at  every  stage  of  it  I  have  had  the  advantage  of 
}'our  cordial  support  and  wise  counsel. 

Very  respectfully,  your  obedient  servant, 

GEO.  F.  BECKER, 
*  -  Geologist-in-charge. 

'See  Second  Auuiial  Report  of  the  Director  U.  S.  Geological  Survey,  page  xi. 

(iii> 


PREFACE 


The  field  work  for  this  report  was  begun  in  April,  ]  880,  and  concluded 
in  March,  1881.  In  the  spring  of  1880  the  Census  of  the  Mineral  Indus- 
tiies  West  of  the  Rocky  Mountains  was  placed  in  my  charge  in  addition  to 
nay  duties  as  geologist,  and  occupied  much  of  my  time  both  during  the 
period  of  field  work  in  the  Washok  District  and  since. 

My  assistants  were  as  follows:  Dr.  Carl  Barus,  physicist,  who  was 
invited  at  my  request  to  join  the  Survey  for  the  express  purpose  of  resum- 
ing the  question  of  the  electrical  activity  of  ore  bodies,  a  subject  in  which 
I  had  long  felt  an  interest.  He  also  made  experiments  on  kaolinization, 
and  the  two  chapters  in  this  volume  devoted  to  these  subjects  sufficiently 
attest  how  ably  he  has  conducted  the  investigations  to  which  he  was 
assigned.  Mr.  F.  R.  Reade,  assistant  geologist,  made  a  large  portion  of  the 
collections,  which  embrace  nearly  three  thousand  numbers,  and,  with  Dr. 
Barus,  earned  out  many  of  the  computations  involved  in  the  discussion  of 
the  increment  of  heat.  I  also  contracted  with  Mr.  R.  H.  Stretch  to  assist 
me  in  mapping  the  underground  geology.  Mr.  Stretch  was  for  some  5'ears 
one  of  the  official  surveyors  of  the  Comstock,  and  his  familiarity  with  the 
old  and  inaccessible  workings  was  of  much  assistance.  In  preparing  the 
sections  it  was  necessary  in  many  cases  to  infer  the  structure  of  localities  to 
which  there  was  no  approach  from  that  shown  in  galleries  on  other  planes, 
a  difficult  task  in  which  Mr.  Stretch's  aid  was  also  very  valuable.  I  visited 
almost  every  foot  of  open  ground,  and  the  structural  and  lithological  geology, 
.as  well  as  the  conjectural  portions  of  the  sections,  are  my  own.  Mr.  Stretch 
was  very  zealous  in  the  collection  of  the  specimens  necessary  to  prove  the 
lithology  of  the  sections,  and  forwarded  the  work  of  the  Survey  in  every 
way  in  his  power. 


Vi  PEEFACE. 

The  claim  map  was  prepared  by  Messrs.  Hoffmann  ..&  Craven,  sur- 
veyors, on  contract,  and  the  mine  maps  were  obtained  through  the  same 
firm  from  official  sources.  Some  additions  have  been  made  to  the  claim 
map  by  Mr.  L.  F.  J.  Wrinkle. 

All  the  mine  officers  were  most  courteous  and  offered  every  facility  for 
the  examination,  often  at  great  inconvenience  to  themselves.  Mr.  I.  E. 
James,  superintendent  of  the  Sierra  Nevada,  had  prepared  a  considerable 
number  of  slides,  which,  as  well  as  his  microscope,  he  placed  at  my  disposal. 
Mr.  Forman,  superintendent  of  the  Forman  Shaft;  Captain  Taylor,  super- 
intendent of  the  Yellow  Jacket;  and  Mr.  I.  Requa,  superintendent  of  the 
■  Chollar,  gave  access  to  their  collections,  and  to  their  temperature  observa- 
tions, as  did  also  Mr.  C.  C.  Thomas,  superintendent  of  the  Sutro  Tunnel. 
Mr.  George  J.  Specht,  surveyor,  compiled  the  temperature  observations  of 
the  Tunnel  and  many  other  data,  most  of  which  will  appear  in  another 
volume.  Mr.  Forman  also  presented  the  Survey  with  a  duplicate  collection 
of  the  rocks  encountered  in  sinking  his  shaft,  a  specimen  having  been  taken 
every  five  feet.  Mr.  W.  H.  Patton  gave  me  extraordinary  facilities  in  the 
series  of  miues  (from  the  Union  to  the  Consolidated  Virginia)  under  his  super- 
intendence; and  Mr.  Hugh  Lamb,  foreman  of  the  Consolidated  Virginia  and 
the  California,  spent  much  time  in  exploring  with  the  party,  and  communi- 
cated many  acute  and  valuable  observations  gathered  in  his  long  experience 
on  the  Lode.  In  short,  from  mine  owners  to  common  miners,  an  intelli- 
gent interest  in  the  objects  of  the  Survey  and  a  willingness  to  forward  them 
were  manifested  by  all  concerned.  It  is  believed  that  the  facts  made  out 
with  reference  to  the  occurrence  of  ore  will  prove  of  sufficient  practical 
advantage  to  justify  this  interest. 

The  lithological  illustrations  were  all  drawn  and  colored  under  my  con- 
stant supervision.  The  endeavor  was  to  reproduce  the  objects  with  absolute 
fidelity,  avoiding  even  the  temptation  to  emphasize  characteristic  outlines  or 
tints,  and  the  figures  were  not  considei'ed  complete  so  long  as  an  addition 
or  a  change  could  be  suggested.  The  work  was  put  on  the  stones,  of  which 
no  fewer  than  eighteen  were  requisite,  by  the  same  draughtsman  who  made 
the  drawings,  Mr.  G.  K.  Gardner,  and  the  originals  have  thus  not  suffered 
ill   lithographic  reproduction.     It  is  safe  to  say  that  no  lithographic  illus- 


PREFACE.  vii 

tnitions  were  ever  more  conscientiously  prepared,  and  I  have  metwitli  none 
which  seem  to  represent  microscopical  effects  more  exactly. 

Special  thanks  are  due  from  me  to  Dr.  Barus  and  to  Mr.  J.  P.  Iddings 
(assistant  geologist  on  Mr.  Arnold  Hague's  staff),  with  whom  I  have 
repeatedly  consulted  on  the  subjects  treated  in  Chapters  IV.  and  III., 
respectively.  But  for  the  stimulus  of  their  criticisms  the  proofs  offered 
would  be  less  satisfactory;  and  in  enabling  me  to  meet  the  objections 
which  occurred  to  them,  they  have  placed  me  more  in  their  debt  than  if 
they  had  made  positive  additions  to  the  discussions  of  lithology  and  faulting. 

The  office  work  has  been  done  at  the  American  Museum  of  Natural 
History,  New  York,  that  institution  having  courteously  placed  some  of  its 
admirable  working  rooms  at  the  disposal  of  the  Survey. 

G.  F.  B. 

New  York,  May  6,    1882. 

Oct.    16. — Mr.    Albert  Williams,  jr.,   Statistician  of  the  Survey,  has 

kindly  given  me  the  benefit  of  his  extremely  efficient  assistance  in  the 

proof  correction  of  the  volume. 

G.  F.  B. 


c  o  ]sr  T  E  N  T  s . 


Page. 

Lettbr  of  Transmittal ^ III. 

Preface V. 

Contents IX. 

List  of  Illustrations XI. 

List  of  Atlas-sheets XIII. 

Brief  outline  of  results XV. 

Chapter    I.— The  Comstock  Mines 1 

II. — Previous  investigations  of  the  Comstock  Lode 12 

III.— Lithology 32 

Section  1 .  The  Rocks  of  the  Washoe  District 32 

2.  The  Decomposition  of  the  Rocks 72 

3.  Propylite 81 

4.  Detailed  description  of  slides 91 

Description  of  illustrations 145 

Tables  of  Analyses  and  Assays 152 

IV.— Structural  Results  of  Faulting 156 

V. — The  Occurrence  and  Succession  of  Rocks 188 

VI.— Chemistry 209 

VII. — Heat  Phenomena  of  the  Lode 228 

Section  1.  General  Discussion 228 

2.  Thermal  Survey '  244 

Vni.— The  Lode 266 

IX. — On  the  Thermal  Effect  of  the  Action  of  Aqueous  Vapor  on  Feldspathic  Eock.s  (Kao- 

linization) by  Carl  Barus 290 

X.— On  the  Electrical  Activity  of  Ore  Bodies by  Carl  Barus 309 

XL— Summary 368 

Note  to  Chapter  III.  (on  the  Determinatiou  of  Feldspars  by  Szab6's  Method) 405 

Index  to  the  Mining  Claims 409 

General  Index 413 

(ix) 


LIST    OF    ILLUSTRATIONS. 


Page, 
Plate  I. — Weathered  augite-andesite  on  divide  between  Mimiit  Rose  and  Monnt  Kate;  Mount 

Davidson  and  Mount  Hutler  in  the  distance  Frontispiece. 

II. — Single  minerals,  as  seen  under  the  microscope follows  151 

III.— Ditto do . . .  151 

IV. — Eock  sections,  as  seen  under  the  microscope do . . .  151 

v.— Ditto do...  151 

VI. — Bullion  Ravine,  looking  east  (diorite);  Mount  Kate  in  the  middle  distance laces  192 

VII. — East  flank  of  Monnt  Rose  (later  homblende-andesite) do.  204 

Fig.    1. — Area  of  extreme  decomposition 73 

2. — Orientation  of  slides J I45 

S. — System  of  equal,  vertical,  movable  sheets 160 

4. — Calculated  and  observed  curves 167 

5. — Logarithmic  curve  referred  to  rectangular  coordinates 168 

6. — Logarithmic  curve  referred  to  inclined  coordinates , 171 

7. — Inclination  of  a  faulted  surface 172 

8. — Double  faul  t-curve I73 

9. — Fault  accompanied  by  a  strain I75 

10. — Contour  map  of  a  faulted  surface 177 

11. — Rise  of  the  hanging  wall 179 

12. — Ideal  section  across  the  Virginia  Range 243 

13. — Combination  Shdf t  and  Yellow  Jacket  temperatures 249 

14. — Yellow  Jacket  Shaft  and  Forman  Shaft  temperatures 251 

15. — Forman  Shaft  temperatures . .  .■- 253 

16. — Rose  Bridge  Colliery  and  Forman  Shaft  temperatures 255 

17. — Spereuberg  Boring,  Forman  Shaft,  and  Rose  Bridge  Colliery  temperatures 257 

18. — Sutro  Tunnel  temperatures 259 

19. — Boiler  used  in  kaolinization  experiments 292 

20. — Disposition  of  apparatus 295 

21. — Section  of  key 29ti 

22. — Ideal  line  of  electrical  survey . .' 316 

23. — Longitudinal  section  of  terminal  ...^ 325 

24. — Terminal  in  jjosition 326 

25. — Mode  of  suspending  wire 327 

26. — Vertical  section  through  Richmond  ore  bodies 331 

27. — Plan  of  the  400  and  500-foot  levels,  Richmond  mine 333 

28. — Earth-potential  and  distance,  Richmond  mine,  400  and  500-foot  levels 343 

29. — Plan  of  the  600-foot  level,  Richmond  mine 344 

30. — Earth-potential  and  distance,  Richmond  mine,  600-foot  level 349 

31. — Disposition  of  apparatus  for  measuring  earth-potential 353 

32. — Earth-potential  and  distance,  Richmond  mine,  600-foot  level  (later  results) 354 

33. — Potential  of  intermediate  points 362 

(xi) 


LIST    OF    ATLAS-SHEETS. 


Sheet. 

Title I- 

Contents II. 

Map  of  the  Washoe  District,  showing  mining  claims III. 

Geological  map  of  the  Washoe  District , IV. 

Vertical  cross-sections  of  the  Comstock  Lode  through  the  Utah,  Sierra  Nevada,  Union,  and  C. 

&C.  shafts V. 

Vertical  cross-sections  of  the  Comstock  Lode  through  the  Sutro  Tunnel  and  the  Porman  Shaft . . .  VI. 
Vertical  cross-sections  of  the  Comstock  Lode  through  the  Combination,  Yellow  Jacket,  Belcher, 

and  Savage  shafts .- VII. 

Horizontal  cross-section  of  the  Comstock  Lode  on  the  Sutro  Tunnel  level  (1,900  feet),  north  end.  VIII. 

Ditto,  south  end -  -  -  IX. 

Vertical  longitudinal  projection  of  the  Comstock  Lode,  showing  the  position  of  ore  bodies  from 

the  Utah  to  the  Potosi X. 

Ditto,  from  the  Bullion- Ward  to  the  Baltimore  Consolidated XI. 

Ditto,  from  the  Overman  to  the  Silver  Hill XII. 

Comstock  Mine  Maps :  No.  1,  Utah,  Sierra  Nevada XIII. 

Ditto :  No.  2,  Sierra  Nevada,  Union,  Mexican XIV. 

Ditto :  No.  3,  Ophir,  California,  Consolidated  Virginia,  Best  &  Belcher XV. 

Ditto :  No.  4,  Gould  &  Curry,  Savage,  Hale  &  Norcross,  ChoUar XVI. 

Ditto :  No.  5,  Potosi,  Bullion,  Exchequer,  Alpha,  Imperial XVII. 

Ditto :  No.  6,  Yellow  Jacket,  Kentuck,  Crown  Point,  Belcher XVIII. 

Ditto :  No.  7,  Segregated  Belcher,  Overman,  Caledonia,  New  York XIX. 

Ditto:  No.  8,  Lady  Washington,  Alta,  Justice,  Woodville,  Silver  Hill,  Succor,  Niagara .'...  XX. 

Ditto :  No.  9,  Knickerbocker,  Baltimore  Consolidated XXI. 

(xiii) 


BRIEF  OUTLINE  OF  RESULTS. 


The  economical  importance  of  the  Comstock  Lode  appears  from  the  fact  that  in  twenty-one  years  a  little  over 
$306,000,000  worth  of  bullion  has  been  extracted  from  it.  Of  this  about  $132,000,000  worth  was  gold.  The  mines  are  the 
deepest  in  America,  reaching  a  distance  of  over  3,000  feet  from  the  surface,  and  they  contain  about  185  miles  of  galleries. 

Besides  the  scientific  importance  attaching  to  the  occurrence  of  the  immense  accumulation  of  ore,  the  Lope  and 
District  present  other  features  of  great  interest.  The  nature  of  the  rocks  associated  with  the  ores,  some  points  of  struct- 
ure, and  even  the  character  of  the  deposit,  have  received  different  explanations  at  the  hands  of  different  observers.  A 
digest  of  the  memoirs  of  Messrs.  von  Richthofeu,  King,  Zirkel,  and  Church  forms  one  chapter  of  the  volume. 

The  subject  of  rock  decomposition  has  received  especial  attention  in  the  examination  described  in  this  report. 
This  study  has  led  to  some  lithological  and  mineralogical  observations  of  interest,  and  to  the  identification  of  all  of  the 
Washoe  rocks  with  well-established  rock  species.  The  greater  part  of  the  hanging  wall  of  the  Lode  is  diabase ;  the  "black 
dike"  is  also  a  variety  of  diabase,  and  the  supposed  trachyte  of  the  District  is  a  homblende-andesite.  The  so-called 
propylite  of  "Washoe  comprises  a  number  of  Tertiary  and  pre-Tertiary  rocks,  reduced  to  a  nearly  uniform  appearance  by 
decomposition.  The  erroneous  determination  of  these  altered  rocks  as  an  independent  species  arose  mainly  from  a  confusion 
between  green  and  fibrous  hornblende  and  chlorite.  The  supposed  propylites  from  the  other  districts  in  the  United  States, 
microscopical  determinations  of  which  have  been  published,  were  also  examined  and  found  to  afford  no  sufficient  evidence 
of  an  independent  rock  species. 

A  discussion  of  faulting  leads  to  an  explanation  of  the  similarity  of  the  shape  of  the  west  wall  of  the  Lode  and  the 
form  of  the  adjoining  face  of  the  Virginia  range.  The  ravines  of  the  latter  are  a  direct  result  of  faulting,  and  are  only 
slightly  modified  by  erosion.  A  cross-section  of  the  country  on  the  Sutro  Tunnel  line  shows  that  the  surface  forms  a 
logarithmic  curve  in  accordance  with  the  theory,  which  is  further  supported  by  experiments.  The  sheeted  structure  of  the 
country  seems  to  be  referable  to  faulting  and  not  to  eruptive  bedding.  The  theory  leads  to  rules  applicable  in  prospecting 
disturbed  but  not  greatly  eroded  districts.  The  details  of  the  topography  of  grassy  hills  are  chiefly  due  to  landslips,  which 
come  under  the  law  of  faults  in  a  modified  form,  and  the  characteristic  curves  of  smooth  hill-slopes  are  logarithmic. 

The  order  of  succession  of  rocks  in  the  Washoe  District  is:  Granite,  metamorphics,  granular  diorite,  porphyritic 
diorite,  metamorphic  diorite,  quartz-porphyry,  earlier  diabase,  later  diabase,  earlier  hornbleude-andesite,  augite-andesite, 
later  homblende-andesite,  and  basalt.    Homblende-andesite  thus  followed  as  well  as  preceded  augitc-andesite. 

Chemical  evidence  is  offered  to  show  that  the  pyrite  of  the  region  is  a  result  of  the  action  of  soluble  sulphides  on  the 
ferro-magrtesian  silicates  of  the  rocks.  Chlorite  is  held  to  be  a  product  of  the  decomposition  of  hornblende,  augite,  or  mica 
while  epidote  forms  at  the  expense  of  chlorite  under  certain  conditions,  but  never  from  feldspar.  There  is  extremely  little 
kaolinization  at  "Washoe,  the  feldspars  having  yielded  to  another  kind  of  decomposition  The  diabase  of  the  hanging  wall 
when  fresh  was  argentiferous  and  auriferous,  and  the  precious  metals  of  the  Lode  are  traced  to  this  rock  with  much 
probability,  the  lateral- secretion  theory  being  thus  affirmed.  It  is  further  supported  by  the  dependence  of  the  other  ore 
bodies  of  the  District  on  the  character  of  the  inclosing  rock. 

The  hypothesis  that  the  heat  of  the  Lode  is  due  to  the  kaolinization  of  feldspar  is  not  confirmed  either  by  theory 
or  experiment.  On  the  other  hand,  there  is  much  geological  evidence  pointing  to  a  deep-seated  source  of  heat,  probably  of 
volcanic  origin.  This  conclusion  is  confirmed  by  extensive  temperature  observations,  from  which  it  appears  that  from  the 
surface  downwards  the  increase  of  heat  is  uniform,  about  1*^  F.  for  every  33  feet,  while  in  a  horizontal  direction  the  heat 
decreases  in  a  geometrical  ratio  to  the  distance  from  the  Lode. 

Experiments  on  the  kaolinization  of  feldspathic  rock,  conducted  at  the  boiling  point  of  water  and  extending  over  a 
number  of  weeks,  show  that  no  heating  effect  due  to  this  cause  could  be  detected  with  an  apparatus  delicate  enough  to 
j'egister  a  change  of  temperature  of  Qo.OOI  C. 

The  numerous  geological  sections  are  discussed  in  Chapter  Vm.,  and  the  application  of  the  explanations  suggested 
in  the  preceding  chapters  is  there  shown  in  detail.  All  the  important  and  profitable  ore  bodies  of  the  Combtock,  it  appears, 
have  occurred  at  or  close  to  the  west  face  of  the  earlier  diabase ;  and  it  is  near  that  surface,  and  there  only,  that  exploration 
is  at  all  likely  to  be  successful.  The  mode  of  occurrence  of  bonanzas  is  considered,  and  hopeful  prognostications  are  made 
for  at  least  two  portions  of  the  Lode;  bnt  a  series  of  bonanzas  nearly  on  the  same  level,  such  as  was  found  in  the  east 
vein  near  the  surface,  is  not  likely  to  recur. 

Electrical  surveys  were  made  both  on  the  Comstock  and  at  Eureka.  At  Virginia  only  negative  results  were 
obtained.  At  Eureka  a  distinct  though  smMl  difference  of  potential  occurs  near  ore  bodies,  and  with  sufficiently  delicate 
'  apparatus  the  method  might  there  be  used  for  prospecting.  It  is  believed  that  sulphuret  ores  would  have  given  results 
of  a  more  convenient  magnitude  than  the  carbonate  ores  of  Eureka. 

(XV) 


GEOLOGY  OF  THE   COMSTOCK  LODE   AND 
THE  WASHOE  DISTRICT. 


BY  GEORGE   F.  BECKER. 


CHAPTER       I. 

THE  COMSTOCK  MINES. 

Importance  of  the  Comstock  mines. The  geologj  of   the    COMSTOCK  LODE,   thoUgh 

of  great  interest  from  a  purely  scientific  point  of  view,  derives  its  chief 
significance  from  the  economical,  industrial,  and  technical  importance  of 
this  extraordinary  ore-deposit.  The  yield  of  the  Comstock  is  supposed  to 
have  exerted  a  seriously  disturbing  influence  on  the  monetary  system  of  the 
civilized  world,  and  its  treasures  have  been  exploited  with  an  unexampled 
rapidity.  It  is  the  chief  focus  of  mining  activity  in  the  region  west  of  the 
Rocky  Mountains,  and  represents  the  most  highly  organized  phase  of  tech- 
nical mining  which  has  been  reached  west  of  the  Mississippi  River. 

The  present  report  deals  exclusively  with  the  geology  of  the  Lode, 
and  of  so  much  of  the  surrounding  country  as  is  supposed  necessary  to  a 
full  comprehension  of  the  occurrence  of  ore.  The  Geological  Survey,  how- 
ever, will  issue  other  volumes  dealing  with  the  Comstock  from  different 
points  of  view.  Mr.  Eliot  Lord  has  prepared  a  report  upon  the  history  of 
mining  on  the  Comstock;  and  Mr.  "W.  R.  Eckart  has  in  preparation  a  volume 
on  the  mining  machinery  in  use.  The  volumes  now  being  prepared  by 
members  of  the  Survey  on  the  census  of  the  mineral  industries,  will  also 
contain  much  technical  information  concerning  the  mines  of  the  Lode. 
Some  of  the  readers  of  the  present  report,  however,  are  unlikely  to  refer 

to  the  other  volumes  relating  to  the  subject,  and  to  them  a  few  introductory 
1  c  L  1 


2  GEOLOGY  OF  THE  COMSTOCK  LODE. 

remarks  setting  forth  in  the  briefest  possible  manner  some  of  the  most  impor- 
tant facts  concerning  the  mines  may  be  of  interest. 

Geographical  position. — The  CoMSTOCK  LoDE  hes  on  the  eastern  slope  of  the 
Virginia  Range,  a  northeasterly  offshoot  from  the  Sierra  Nevada.  From 
Mount  Davidson  the  snow-capped  peaks  of  the  Sierra  can  be  seen  stretching 
far  away  to  the  southeast,  their  flanks  partially  covered  with  trees;  but  to  the 
east  and  northeast  lies  the  desert  region  of  the  Great  Basin,  visible  through 
the  clear  air  for  a  hundred  and  fifty  miles.  Comparatively  low  ranges, 
running  north  and  south,  break  the  surface  of  the  Great  Basin  at  short 
intervals,  and  as  seen  from  Virginia  these  appear  in  seemingly  endless  suc- 
cession, like  the  waves  on  a  stormy  sea.  They  are  clothed  only  by  the  low 
growing,  gray-green  desert  shrubs  known  as  "sage-brush,"  and  every  detail 
of  the  mountain  sculpture  is  visible  through  the  vaporless  atmosphere  at 
great  distances.  White  alkali  deserts  appear  here  and  there  in  the  valleys, 
and  now  and  then  one  catches  a  glimpse  of  the  Carson  River,  which  dwin- 
dles almost  from  its  source,  and  is  at  last  wholly  absorbed  in  the  parched 
earth.  The  Great  Basin,  which  is  five  hundred  miles  wide,  is  bounded  to 
the  east  and  west  by  high  ranges.  During  the  greater  part  of  the  year  these 
mountains  precipitate  almost  all  the  moisture  from  the  air-currents  passing 
over  them,  and  at  certain  stations  in  the  Basin  ordinary  meteorological 
instruments  sometimes  fail  to  show  any  moisture  in  the  air. 

The  parallelism  of  structure  expressed  by  the  disposition  of  the  ranges 
in  California  and  the  Great  Basin  finds  a  correspondence  in  the  distribution 
of  metalliferous  minerals,  as  was  long  since  pointed  out  by  Prof  W.  R 
Blake.  The  coast  ranges  of  California  carry  quicksilver,  coal,  and  chromic 
iron.  On  the  western  slope  of  the  Sierra  Nevada  is  a  lower  belt  of  copper 
deposits,  and  a  higher  and  more  easterly  one  of  gold.  Along  the  eastern 
base  of  the  Sierra  is  a  zone  of  silver  deposits,  the  richest  known  point  of 
which  is  the  Comstock,  while  still  farther  east  in  the  Great  Basin  there  are 
less  sharply  defined  belts  carrying  comj)lex  silver  ores  and  argentiferous 
lead. 

Difficulties  of  mining. — Miulug  ou  the  CoMSTOCK  begau  in  1859,  and  has  been 
carried  on  ever  since,  but  only  in  spite  of  obstacles  of  the  most  formidable 
character.     Only  the  scantiest  supplies  of  potable  water  existed  on  the  spot, 


THE  COMSTOCK  MINES.  3 

and  that  obtained  from  the  mines  was  not  fit  even  for  the  production  of  steam. 
After  many  diificulties  this  want  was  overcome  by  laying  lines  of  pipe  to  a 
source  in  the  Sierra  Nevada,  25  miles  from  the  Lode,  at  a  cost  of  $2,200,- 
000.  Up  to  1 870  not  only  all  the  machinery,  but  almost  all  the  food  of  the 
settlement  was  transported  by  wagon  from  beyond  the  Sierra,  mainly  from 
Sacramento,  a  distance  of  165  miles.  The  freight  charges  were  of  course 
enormous;  in  the  earhestdays  as  high  as  fifty  cents  a  pound;  but  later  from 
five  to  ten  cents.  The  Carson  Valley,  however,  furnished  a  small  portion  of 
the  necessary  food  supply.  In  1870  a  branch  railroad  from  the  Central 
Pacific  was  completed.  The  junction  is  at  Reno,  22  miles  from  Virginia; 
but  the  railway  connecting  the  two  points  is  52  miles  in  length,  a  fact  which 
indicates  the  character  of  the  country  through  which  it  passes.  Fuel  and 
timber  are  obtained  from  the  Sien'a  at  points  from  10  to  30  or  more  miles 
distant;  but  transportation  down  the  slopes  of  the  range  is  efi"ected  in  flumes 
by  water  with  a  great  saving  of  expense.  The  difficulties  to  be  overcome 
in  mining  on  the  Comstock  were  not  less  formidable  than  those  met  with  in 
establishing  a  settlement.  The  ground  has  been  in  great  part  very  bad,  th.e 
size  of  the  ore-bodies  required  the  development  of  a  new  system  of  timbex'- 
ing,  and  floods  have  burst  into  the  mines  which  it  took  years  to  drain ;  but 
by  far  the  greatest  obstacle  has  been  the  heat,  which  increases  about  3° 
Fahrenheit  for  every  additional  hundred  feet  sunk,  and  which  seems  likely 
eventually  to  put  an  end  to  further  sinking.  According  to  Mr.  Church,  the 
amount  of  air  passing  through  the  mines  is  nearly  300,000  cubic  feet  a 
minute,  while,  except  at  the  change  of  shift,  there  are  probably  never  1,000 
men  below  ground  ;  3^et  there  are  few  spots  where  the  miners  can  work  more 
than  each  alternate  hour  during  the  eight  hours'  shift,  so  that  double  gangs 
to  relieve  each  other  are  practically  always  necessary,  and  at  many  points 
the  conditions  are  still  more  disadvantageous.  Besides  every  alleviation 
which  artificial  ventilation  can  afford,  the  men  must  also  be  supplied  with 
unlimited  quantities  of  ice-water  both  for  drinking  and  washing.  With  all 
these  unheard-of  easements,  many  men  have  died  from  overheating,  and 
some  from  contact  with  scalding  water.  Many  more  have  fainted  while 
coming  to  the  surface  on  the  cages  when  they  met  the  cool  air,  and  have 


4  GEOLOGY  OF  THE  COMSTOCK  LODE. 

been  dashed  to  pieces  in  the  shafts.^  None  of  the  miners  in  the  hot 
mines  receive  less  than  $4  a  day  (eight  hours),  and  a  few  get  more. 

Good  condition  of  the  miners. — In  spitc  of  the  trying  conditions  the  men  are,  with 
very  rare  exceptions,  in  excellent  physical  condition.  This  appears  to  be 
attributable  to  two  causes.  Even  those  who  desire  to  practice  close  econ- 
omy find  themselves  unable  to  live  on  the  coarse  fare  on  which  miners  in 
other  districts  frequently  subsist.  The}'  must  have  not  only  fresh  meat  but 
fruit  at  any  cost,  and  are  large  consumers  of  raw  oysters  brought  from  San 
Francisco  on  ice,  and  similar  delicacies.  In  short,  they  are  compelled  by 
the  physical  effects  of  the  conditions  to  which  they  are  exposed,  to  employ 
a  much  better  diet  than  most  workingmen.  Moreover,  while  in  the  mines, 
they  are  almost  constantly  in  a  perspiration  as  profuse  as  that  induced  by 
a  Turkish  bath,  a  condition  almost  incompatible  with  bilious  disorders. 
They  are  thus  much  less  liable  than  other  workmen  to  derangements  of  the 
digestive  system,  and  are  well  nourished  and  extremely  vigorous.  The 
average  weight  of  the  men  is  166  pounds.  It  is  said  that  short  as  the  hours 
of  labor  are,  the  work  accomplished  per  man  is  as  great  as  in  cool  mines. 
In  the  California  in  1877  the  average  amount  of  ore  raised  per  man,  includ- 
ing employes  of  every  kind,  was  1.13  tons  per  day. 

Population. — The  average  number  of  miners  employed  from  1860  to  1870 
was,  as  nearly  as  can  be  ascertained,  about  1,500.  From  1870  to  1880  it 
was  probably  as  high  as  3,200,  but  in  January,  1880,  the  number  had 
fallen  off  to  2,770.  The  population  of  the  towns  of  Virginia,  Gold  Hill, 
and  Silver  City  has  fluctuated  greatly  with  the  condition  of  the  mines 
and  the  number  of  miners  at  work.  Silver  City  has  never  had  many  inhab- 
itants, while  Gold  Hill  and  Virginia  long  since  extended  over  the  space 
which  originally  separated  them,  and  are  divided  only  by  artificial  lines.  In 
round  numbers  the  population  of  the  three  settlements  in  1860  was  4,000; 
in  1870,  13,000;  and  in  1880,  15,500.  The  maximum  number  of  inhab- 
itants thus  far  was  about  21,000  in  the  year  1876. 

'  It  may  not  be  improper  to  remark  that  geological  examinations,  which  cannot  of  course  be  con- 
fined to  actual  workings  where  everything  possible  is  done  to  keep  the  air  good,  are  exceedingly  trying. 
All  the  members  of  my  jiarty  were  at  times  more  or  less  overcome  by  heat  and  bad  air.  I  once  fainted 
on  the  cage,  and  owe  my  life  to  the  firm  grasp  of  Mr.  Hugh  Lamb,  foreman  of  the  Consolidated  Virginia 
and  California  mines. 


THE  COMSTOCK  MINES.  5 

School  statistics. — It  would  be  easy  to  illustrate  the  wild  life  characteristic 
of  the  mining  camps  of  the  far  West  by  citing  the  liquor  consumption  of 
Virginia  and  Gold  Hill,  or  the  statistics  of  gambling,  which  is  a  legal  occu- 
pation in  the  State  of  Nevada;  but  it  is  pleasanter  and,  in  some  respects,  more 
just,  to  turn  to  the  school  statistics  of  these  towns.  The  methods  employed  in 
the  primary  and  grammar  schools  appeared  to  me  fully  equal  to  those  in  use 
in  the  larger  cities  of  the  Union,  and  the  results  reached  at  least  as  good.  The 
proportion  of  children  attending  school  is  cei'tainly  remarkable,  when  it  is 
considered  that  of  those  reported  as  not  attending  either  public  or  private 
schools  a  very  large  number  must  be  considered  by  their  parents  too  young 
to  be  sent,  while  many  more  have  left  school  after  a  number  of  years' 
instruction.     The  official  figures  for  Storey  County  are  as  follows: 


School  attendance. 


1880. 


Number  of  children  between  6  and  18  years  not  attending  school 

Nnmber  of  children  between  6  and  18  years  represented  as  attending  private  schools  . 
Number  of  children  between  6  and  18  years  represented  as  attending  public  schools  . . 


211 
493 


763 

543 

2,565 


The  number  of  boys  and  girls  in  the  schools  is  very  nearly  equal.  The 
proportion  of  children  to  adults  is  of  course  far  smaller  in  these  towns  than 
in  ordinary  settlements,  a  very  large  part  of  the  miners  being  unmarried,  and 
some  having  families  elsewhere. 

Extent  of  the  mines. — The  total  length  of  gallerics  and  shafts  on  the  Comstock 
up  to  January,  1881,  is,  as  nearly  as  can  be  ascertained,  between  180  and 
190  miles.  Of  this  about  154  miles  is  a  matter  of  record  on  the  official 
maps,  but  though  all  more  important  galleries  are  run  by  survey  and  plotted 
on  the  maps,  many  drifts  of  subordinate  importance  are  cut  without  the  help 
of  the  surveyor.  These  are  estimated  at  a  total  of  30  miles,  after  consulta- 
tion with  surveyors  and  superintendents.  An  immense  consumption  of  tim- 
ber is  a  necessity  of  mining  on  the  Comstock.  This  is  due  to  the  shifting 
character  of  much  of  the  ground,  to  the  great  size  of  the  ore  bodies,  and  to  the 
necessity  of  keeping  a  large  extent  of  workings  open  to  secure  rapid  ventila- 
tion, and  as  great  a  diminution  of  temperature  as  practicable.  The  timbers 
are  all  sawn  square,  the  commonest  size  being  12  by  12  inches.  They  are  cut 
in  lengths  and  the  ends  fitted  in  shops  on  the  surface,  and  they  are  placed 
underground  without  the  use  of  nails.     The  system  is  described  in  Mr.  J, 


6  GEOLOGY  OF  THE  COMSTOCK  LODE. 

D.  Hague's  admirable  memoir,  "The  Comstock  Mines,"^  and  has  undergone 
no  essential  modification  since  the  date  of  that  work.  The  consumption  of 
timber  in  the  mines  up  to  the  close  of  1880  is  estimated  at  450,000,000 
board  feet. 

The  only  fuel  used  on  the  Comstock  is  wood,  derived  from  the  same 
sources  as  the  timber.  The  larger  part  reaches  the  town  by  rail,  but  a  con- 
siderable quantity  is  floated  down  the  Carson  River  to  convenient  points, 
and  hauled  to  Gold  Hill  in  wagons.  The  consumption  of  fuel  at  the  mines 
in  hoisting  and  pumping  is  increasing  rapidly,  for  the  quantity  of  water  is 
greater  year  by  year,  as  well  as  the  distance  through  which  it  must  be 
forced.  During  the  census  year,  ending  May  31 ,  1880,  about  1 10,000  cords 
were  burned;  and  from  1860  to  1880  the  consumption  cannot  have  been 
less  than  about  900,000  cords.     The  mills  have  burned  about  as  much. 

Milling. — In  the  early  days  of  mining  on  the  Comstock  considerable  quan- 
tities of  very  rich  and  complex  ores  occurred,  and  these  were  treated  by  roast- 
ing and  barrel-amalgamation.  Later  the  ores  became  more  facile,  and  the 
system  of  pan- amalgamation  was  developed  and  applied  with  success.  For 
many  years  it  has  been  found  practicable  to  beneficiate  all  the  ores  met 
with  by  this  process,  with  the  aid  of  "bluestone"  (cuprous  sulphate)  and 
salt.  The  success  of  the  process  is  unquestionably  due  in  a  large  measure 
to  the  chemical  activity  of  the  iron.  Formerly  the  mills  guaranteed  a  return 
of  65  per  cent,  of  the  assay  value  of  the  ores,  but  of  late  years  72  per  cent, 
is  guaranteed,  and  above  80  per  cent,  is  often  returned.  The  slimes  and 
tailings  belong  to  the  mills,  which  work  them  up  for  their  own  account  or 
sell  them  from  time  to  time  to  other  mills  having  especial  facilities  for  their 
treatment.  Tailings  not  caught  by  the  mills  and  deposited  at  considerable 
distances  in  the  streams  have  also  been  treated  with  success  in  a  small  way. 
On  the  whole,  however,  it  is  improbable  that  more  than  75  per  cent,  of  the 
bullion  contained  in  the  ore  has  been  recovered  from  it,  and  it  is  therefore 
fair  to  estimate  that  the  ore  received  has  contained  at  least  four  hundred 
million  dollars,  of  which  about  three-quarters  has  reached  the  market. 

Relative  quantities  of  gold  and  silver  produced. The    qUestioU    of    the     prOpOrtioU    of 

gold  to  silver  in  the  Comstock  bulHon  is  one  of  considerable  importance  in 

'  Exjjloration  of  the  Fortieth  Parallel,  Vol.  III. 


THE  OOMSTOCK  MINES.  7 

discussions  upon  the  price  of  silver  and  kindred  subjects.  It  has  often  been 
assumed  that  the  product  of  these  mines  is  almost  wholly  silver,  but  as  will 
appear  from  the  tables  it  would  be  much  neai-er  the  truth  to  assume  that  the 
Lode  yielded  an  equal  value  of  each  of  the  precious  metals.  I  find  that  the 
published  and  accessible  mine  reports  give  the  assay  values  of  nearly  two- 
thirds  of  the  total  product,  and  there  is  every  reason  to  suppose  that  Baron 
V.  Richthofen's  estimate,  made  when  the  total  product  was  comparatively 
small  and  very  recent,  was  a  very  close  approximation  to  the  truth.  Some 
of  the  mining  companies  reported  only  the  total  value  of  bulhon  produced; 
and  others  gave  the  gold  and  silver  assays  in  some  years,  but  not  in  others, 
or  only  for  certain  lots  of  bullion.  The  figures,  however,  cover  portions  of 
all  the  important  ore  bodies  excepting  that  in  the  Justice,  and  it  appears 
certain  that  not  far  from  57  per  cent,  of  the  product  of  the  Lode  has  been 
silver,  or,  say,  $174,000,000,  and  that  43  per  cent,  or  $132,000,000,  has 
been  gold.  The  table  from  which  this  conclusion  is  drawn  is  given  in  con- 
siderable detail,  chiefly  for  the  purpose  of  showing  the  differences  in  the 
ratio  of  gold  to  silver  in  the  various  mines.  In  the  Belcher,  for  the  time 
over  which  the  record  extends,  about  57  per  cent,  of  the  value  of  the  bullion 
produced  was  in  gold,  while  in  the  Yellow  Jacket  only  about  31  per  cent, 
was  in  gold,  Even  in  the  great  bonanza  of  the  Consolidated  Virginia,  Cali- 
fornia, and  Ophir  mines,  the  California  or  central  portion  of  the  body  was 
far  richer  in  gold  than  the  northern  and  southern  ends. 

The  table  of  production  is  due  to  Mr.  Eliot  Lord,  who  has  taken  great 
pains  to  sift  the  records  and  to  ascertain  the  truth  as  closely  as  is  now 
practicable.  This  and  the  other  appended  tables  need  no  further  explana- 
tion. 


8 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


SUPPLIES  BROUGHT  TO  THE  COMSTOCK  TOWNS  DURING  THE  CALENDAR  YEAR  1879,  AND  ESTIMATED 

CONSUMPTION. 


Character. 

Unit. 

Total 
amount. 

Mine  use. 

Mill  use. 

Permanent 
construc- 
tion. 

Domestic 
use. 

Cords 

185,  6a2J 

31,  443,  771 

2,  373,  919 

183,  366 

725,  092 

520,  319 

51,  594 

5,  910,  355 

462,442 

32,  514 

1,  003,  808 

29,251 

119,207 

16,  672 

110,  000 
23,  000,  000 
873,  919 
122,  244 
725,  092 
520,  319 
51,  594 

40,000 

35,  622J 
6,  443.  771 

Board  feet 

1,  500,  000 
61, 122 

Steel 

....do     

...do 

....do     

! 

....do  



..  do     

5,910,355 

462,442 

32,  514 

1,  003,  808 

Nails          .                  

....do  

Nut8 

.    do     

do 

do      ... 

27, 231 
89,  405 
11,115 

1,000 
29,  802 
5,557 

1,000 

Gallons 

do     

ADDITIONAL,  USED  BT  THE  MILLS. 


QolcksilTer . 
Bluestone... 
Salt 


Pounds . 
...do... 
...do  ... 


300, 000 

2,  500,  000 

450, 000 


MINE  AND  MILL  SUPPLIES  CONSUMED  ON  THE  COMSIOCK  DURING  THE  CALENDAR  TEAR  1879. 

COST. 


Character. 


Wood 

Timber 

Iron 

Steel 

Candles 

Explosives 

Quicksilver 

Salt 

Bluestone 

Lard  oil 

Lubricating  oil . 
Sundries 


Total. 


Mine  use. 


$1, 100,  000  00 

500,  000  00 

52,435  14 

22,  003  92 

123,265  64 

208, 127  60 


89,  405  00 

4,  446  00 

♦106, 149  00 


2,  205, 832  30 


Mill  use. 


$400,  000  00 


90,  000  00 
11,  001  96 


135,  000  00 
25;  000  00 
45,  000  00 
29,  802  00 
2, 222  80 

t75,  000  00 


813,  026  76 


Total. 


$1,  500,  000  00 

500,  000  00 

142, 435  14 

33,  005  88 

123,  265  64 

208, 127  60 

135,  000  00 

25,  000  00 

45,  000  00 

119,  207  00 

6,  668  80 

181, 149  00 


t3,  018,  859  06 


*  Including  ice,  "water,  charcoal,  coal  oil,  stone  coal,  tools,  etc. 

t  Including  water,  tools,  lights,  chemicals,  etc. 

JDoes  not  include  machinery,  etc.,  entering  into  permanent  construction. 


THE  COMSTOCK  MINES. 


9 


PBOPOETIONS  OF  GOLD  AND  SILVER  IN  COKSTOCK  BULLION. 

[From  official  reports  of  the  mining  companies,  so  far  as  accessible.    The  product  is  not,  in  all  cases,  thus  segregated  into 
gold  and  silver  in  the  companies'  reports.    The  figures  quoted  are  of  assay  (not  market)  values.] 


Gold. 

Silver. 

Total. 

Percentage. 

Source. 

Gold. 

Silver. 

GOLD  HILL  GKOUP. 

Crown  Point,  from  May  1,1864,  to  May  1,1877 

Belcher,  from  January  1, 1871,  to  December  31, 1873. . . 

$10, 166,  656  88 
8, 813, 196  06 

170. 133  13 
1,  973,  021  60 

563, 121  83 

$13,  762,  812  77 
6,  716,  231  05 

363, 123  80 
2,  588, 138  85 

786,713  69 

$23,929,469  65 

15,  529,  427  11 

533,  256  92 

4,  561, 160  45 

1,  349,  836  52 

Empire,  from  December  21, 1864,  to  December  16, 1868. 

21,  686, 129  49 

24,  217,  020  16 

45,  903, 149  65 

47.25 

52.75 

CENTRAL  GROUP. 

3,  661,  220  70 

577,  729  22 

2,  772, 468  28 

3,868,<88  14 

7,  090,  573  61 
1,  219, 113  16 
4,  774, 187  26 
6,  314,  261  66 

10,  751,  794  31 
1,  796,  842  38 
7,  546,  655  54 

10, 182,  749  80 

Gould  &  C urry,  December  1, 1865,  to  November  30, 1867 . 
Hale  &  Norcross,  March  1. 1866,  to  January  31, 1874. . . 

10,  879,  908  34 

19,  398, 135  69 

30,  278,  042  03 

35.93 

64.07 

"BONANZA"  GROUP. 

29,  075,  338  97 
23,308,012  69 
2, 172,  600  57 

35.895,438  98 

23,  428,  818  75 

2,  608,  744  28 

64,  970,  777  95 
46,  736,  831  44 
4,  781,  344  85 

64,  555,  952  23 

61,  933,  002  01 

116,  488,  954  24 

46.83 

53.17 

RECAPITULATION. 

21,  686, 129  49 
10,  879,  906  34 
54,  555,  952  23 

24,  217,  020  16 
19,  398, 135  69 
61,  933,  002  01 

45,  903, 149  05 
30,  278,  042  03 
116,488,954  24 

87, 121,  988  06 
15,  250,  000  00 

105,  548, 157  86 
32,  750,  000  00 

192,  670, 145  92 
48,  000,  000  00 

45,22 
31.77 

54.78 

Baron  von  Kiehthofen's  estimate  of  the  yield  of  the 

68.  23 

Xotal                           

102,371,988  06 

138,  298, 157  86 

240,  670, 145  92 

42.54 

57.46 

10 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


BULLION  PEODTTCT  OF  THE  COMSTOCK  LODE  TO  JUNE  30, 1880. 

[So  far  as  ascertainable  from  the  official  reports  of  the  mining  companies  and  the  assessors'  returns.] 

1  ounce  8ilver=$1.2929. 


Mine. 


Date. 


Ore  treated. 


Tons. 
(2,000  lbs.) 


Founds. 


Average 

yield    per 

ton. 


Product. 


Alta 

American 

Andes 

Bacon 

Belcher 

Bowers 

Burke  &  Hamilton 

Caledonia 

California 

Challenge 

Chollar 

ChoUar-Potosi 

Confidence 

Consolidated 

Consolidated  Imperial . 
Consolidated  Virginia. . 

Crown  Point 

Eclipse 

Empire 

Gold  Hill  M.  &M.  Co.. 

Gould  itCurry 

Hale  &  Norcross 

Hartford 

Imperial 

Justice 

Kentuck 

Luzerne 

Mexican 

Midas 

Ophir 

Overman 

Plato 

Potosi 

Savage 

Segregated  Belcher 

Sierra  Nevada 

SUverHill 

Succor 

Trojan 

Union  Consolidated  — 

Woodville 

Yellow  Jacket 

Total 


1879 
1871. 
1875 
1867 
1868 
1867 
1868. 
1871 
1876 
1867 
1879 
1866 
1867 
1867 
1876 
1873 
1864 
1868 
1864 
1867 
1860 
1866 
1871 
1864 
1873 
1865 
1871 
1867 
1871 
1860 
1866 
1868. 
1879. 
1863 
1867 
1868 
1873 
1871 
1877 
1879 
1872 
1864 


to  June  30, 1880. 


to  1878,  inclusive 
to  1869,  inclusive . 
to  June 30, 1880... 
to  1875,  inclusive . 


to  1873.  inclusive . 
to  June  30, 1880... 
to  1873,  inclusive . 


to  1878,  inclusive . 

and  1868 

and  1868 

to  June  30, 1880... 
to  June  30, 1880... 
to  1878,  inclusive . 


to  1877,  inclusive . 
to  1872,  inclusive . 
to  1873,  inclusive . 
to  1875,  inclusive  . 


to  1876,  inclusive  . 
to  1879,  inclusive . 

to  1872  

and  1872 


and  1872 

to  June  30, 1880... 
to  1877,  inclusive . 


to  June 30, 1880... 
to  1871,  inclusive  . 
to  June 30, 1880... 
to  1879,  inclusive . 
to  1873,  inclusive . 
to  1879,  inclusive . 
to  June  30, 1880... 
to  1875,  inclusive . 
to  1876,  inclusive . 


2, 

2, 

22, 

702, 

4, 

1, 

26, 

559, 

1, 

1, 

556, 

10, 

11, 

37, 

784, 

815, 

3, 

162, 

10, 

306, 

320, 

2, 

223, 

183, 

142, 

11. 

1, 
*165, 

77, 


482, 
4, 
119, 
13, 
16, 
12, 
30, 
7, 
443, 


250 


.300 


1,000 

1,450 

1,135 

333 


1,540 


700 

21 

1,010 


256 
230 


1,740 
1,073 
1,220 


1,725 

448 

1,000 


210 
1,000 


750 
175 


$10  38 
14  38 

16  86 
24  99 
46  52 

17  36 

28  22 
12  50 
82  72 

22  56 

14  21 

24  21 

20  74 
42  65 

15  42 
82  26 

36  84 

25  65 

21  05 

23  70 
50  70 

24  91 
9  02 

23  42 
19  40 

34  47 
5  08 

35  32 
9  97 

37  79 

18  18 

19  71 
15  53 
34  32 

20  44 
8  64 

10  53 

10  03 

11  27 

38  84 
17  21 

29  29 


6,  281,  885 


221 


44  26 


$*, 

32, 

42, 

570, 

32,  672, 

83, 

28, 

337, 

46,  278, 

43, 

14, 

13,  471, 

217, 

504, 

583, 

64,  508, 

30,  049, 

81, 

3, 414, 

240, 

15,  525, 
7,  986, 

18, 

5,  224, 

3,554, 

4,905, 

57, 

28, 

12, 

13,  659, 

1, 411, 

15, 

16,  552, 
101, 

1,  035, 
140, 
162, 
144, 

1, 174, 

121, 

12, 998, 


116  66 

705  00 
931  25 
166  29 
245  95 

465  99 
028  10 
999  72 
839  05 
585  02 
917  97 
217  28 
561  49 
012  49 
470  23 
673  50 
431  04 
594  12 
525  67 
no  13 
075  49 
951  18 
672  75 
461  69 
271  01 
853  61 
645  79 
601  53 
622  33 
489  06 
728  47 
124  27 
254  23 

466  81 
363  16 
657  17 
440  51 
392  81 
803  45 
813  33 
170  82 


278.  012,  865  08 


*  Tonnage  from  1860  to  1870  not  ascertainable;  average  stated  is  for  16S,038^JJJ  tons  produced  &om  1874  to  1880,  inclusive. 


THE  COMSTGOK  MINES. 


11 


BnXLION  PRODUCT  OF  OTHEK  MINES  IN  THE  WASHOE  DISTRICT  TO  JUNE  30, 1880. 


Mine. 

Date. 

Ore  treated. 

Average 

yield  per 

ton. 

Product. 

Tons. 
(2,000  lbs.) 

Poands. 

1873  

■  240 
3,425 
1,004 
7,849 
2,161 

$8  11 

18  83 
10  98 

19  25 
15  81 

$1,  948  10 
64,  507  96 

1879  to  Jnne  30, 1880 

Occidental  ...        

1868  to  1873,  inclusive   

151  152  87 

1875aDtll876     

34  165  00 

Total        

14,  679 

17  90 

262  806  93 

BULLION  PRODUCT  EROM  TAILINGS  OF  COMSTOCEl  ORES  TO  JUNE  30,  1880. 
[So  far  as  ascertainable  from  oflScial  reports  of  the  miuing  companies  and  tlie  assessors'  returns.  ] 

1  ounce  silver  =  $1.2929. 


Mine. 

Date. 

Product. 

1868        

$229  97 
4  811  96 

Barke  &  Hamilton                     .          ... 

1868 

1868  and  1869 

45,  406  63 

8,  342  45 

334,  918  94 

1  550  89 

Gold  Hill  M.  &  M.  Co      

1871  and  1872    

Gould  &.  Curry          

1865  to  1869  inclusive 

Hale  &.  Korcroas 

1867  and  1868 

Hartford 

1871 

195  50 

1870 

29,  362  01 
81,  828  94 

1866  to  1870,  inclusive 

1871 

1868  and  1869      

24  983  99 

1871  to  June  30,  1880 

3  765  000  74 

4,  306,  632  02 

RECAPITULATION. 

Product  of  the  lode  traceable  by  mines $278,  012, 865  08 

Product  of  other  mines  in  the  district 262,  806  93 

Tailings 4,  .306, 632  02 


Total  product  directly  traceable 282,  582, 304  J)3 

Estimated  additional  production,  chiefly  in  early  years 23,  598,  947  02 


Total  yield  of  the  Washoe  District  to  June  30,  1880 306,181,251  05 


CHAPTER    II. 

PREVIOUS  INVESTIGATIONS  OF  THE  COMSTOCK  LODE. 

V.  Richthofen's  report. — III  1865,  Baron  Fei'dmand  voH  Richthofen  made  an 
examinatiou  of  the  Comstock  for  the  Sutro  Tunnel  Company,  a  report  of 
which  was  issued  by  that  corporation,  but  never  published  in  the  proper  sense 
of  the  word.^  It  met  the  ordinary  fate  of  mine  reports,  and  is  now  scarcely 
obtainable.  It  was,  however,  a  very  important  contribution  to  American 
geology,  and  no  one  who  has  studied  the  Lode  has  failed  to  acknowledge  his 
indebtedness  to  it.  The  mines  are  now  about  six  times  as  deep  as  at  the 
date  of  von  Richthofen's  examination,  but  his  opinions  and  predictions  have 
for  the  most  part  been  verified  in  a  very  remarkable  manner;  and  had  his 
lithological  determinations  been  as  accurate  as  his  insight  into  structure  was 
keen,  I  should  have  had  little  to  do  beyond  confirming  and  amplifying  upon 
his  views,  in  spite  of  immensely  increased  facilities  for  observation.  In  litho- 
logy,  as  is  well  known,  there  has  been  almost  a  revolution  since  von  Richt- 
hofen wrote,  an*d  nothing  is  less  strange  than  that  some  of  his  determina- 
tions should  fail  to  stand  microscopic  tests.  On  account  of  the  rarity  of 
Baron  von  Richthofen's  report,  I  take  the  liberty  of  reproducing  almost 
entire  and  verbatim  its  geological  portions.  In  these  days  of  diffuse  writing 
its  conciseness  must  be  regarded  as  one  of  its  important  merits. 

Rocks  of  the  district. — The  more  important  rocks  of  the  Washoe  district  are 
as  follows : 

Syenite,  containing  both  orthoclase  and  oligoclase,^  mica  and  epidote, 

'  The  Comstock  Lode :  its  character  and  probable  mode  of  continuance  in  depth.  By  Ferdinand 
Baron  Richthofen,  Dr.  Phil.  (Nov.  22d,  18G5),  San  Francisco:  published  by  the  Sutro  Tunnel  Company. 
Towne  &  Bacon,  printers,  1866.    88  pp.    8vo. 

''Professor  Zirkel  showed  ten  years  later,  by  the  help  of  the  microscope,  that  this  rock  contains 
exclusively  plagioclase,  and  is  therefore  a  diorite. 
12 


PEEVIODS  INVESTIGATIOJfS.  13 

but  no  quartz.  It  forms  the  prominent  topographical  feature  of  the  dis- 
trict, Mount  Davidson.  Adjoining  the  syenite  to  the  north  and  south  are 
metamorphic  rocks,  the  most  recent  of  which  Prof.  J.  D.  Whitney  has  shown 
to  be  Triassic.  Overlying  a  portion  of  the  metamorphic  strata  is  quartzose 
porphyry.! 

The  foregoing  form  the  ancient  series.  Of  the  Tertiary  rocks  only 
two^  have  any  close  relation  to  the  Comstock  Lode.  Propylite  has  this 
remarkable  peculiarity,  namely:  that  it  resembles  many  ancient  rocks 
exactly  in  appearance,  and  yet  is  among  the  most  recent  in  origin.  It  is 
prominent  among  the  inclosing  rocks  of  the  Comstock  vein  and,  besides, 
incloses  several,  perhaps  most,  of  the  largest  and  most  productive  silver  veins  in 
the  world,  as  those  in  the  Karpathian  Mountains,  of  Zacatecas  and  other 
places  in  Mexico,  and  probably  several  veins  in  Bolivia.  Mineralogically 
it  consists  of  a  fine-grained  paste  of  ordinarily  greenish,  but  sometimes 
gray,  red,  and  brown  color,  with  embedded  crystals  of  feldspar  (oligoclase), 
and  columns  of  dark-green  and  fibrous,  seldom  of  black,  hornblende,  which 
is  also  the  coloring  matter  of  the  base.  A  peculiarity  of  the  rock  is  its 
ferruginous  character  when  decomposed.  Probably  it  contains  other  metals 
besides  iron.  Geologically  it  is  an  eruptive  rock,  but  it  is  accompanied  by 
vast  accumulations  of  breccia,  which  is- sometimes  regularly  stratified.  The 
flats  of  Virginia  City,  Gold  Hill,  American  City,  and  Silver  City  consist  of 
propylite.  It  lies,  in  general,  east  of  the  mountains  consisting  of  the  ancient 
formations,  and  contains  several  mineral  veins  besides  the  Comstock  Lode. 
Its  distribution  in  other  countries  of  the  world  is  not  very  general.  Sev- 
eral different  kinds  of  volcanic  and  eruptive  rocks  followed  the  outbreak  of 
propylite,  but  only  one  of  them  demands  attention  in  reference  to  the  Com- 
stock vein,  as  it  probably  caused  its  formation,  besides  taking  a  prominent 
part  in  the  structure  of  the  country.     This  is  sanidin  trachyte. 


iln  his  memoir  ou  The  Natural  System  of  Volcanic  Rocks,  p.  41,  Baron  von  Eichthofen  says: 
"  Quartzose  porphyry  occurs  to  some  extent  in  Washoe  under  circumstances  which  make  the  exact 
determination  of  its  age  difficult,  but  render  it  certain  that  it  is  intermediate  in  this  respect  between 
granitic  and  volcanic  rocks."  This  rock  was  later  regarded  by  Mr.  King  as  quartz-propylite,  and 
determined  by  Professor  Zirkel  as  dacite.     (v.  Quartz-porphyry.) 

=In  his  Natural  System  of  Volcanic  Rocks,  p.  31,  Baron  von  Eichthofen,  speaking  of  the 
Washoe  district,  says:  "Andesite  is  insignificant  in  bulk  in  that  region.  It  composes  a  few  hillocks  on 
the  propylitic  plateau,  and  in  some  cuts  and  tunnels  andesitic  dikes  may  be  seen." 


14  GEOLOGY  OF  THE  COMSTOCK  LODE. 

The  mode  of  occurrence  of  the  trachyte  shows  that  it  has  been  ejected 
through  long  fissures  in  a  viscous  or  hquid  state,  and  at  a  high  tempera- 
ture. In  some  places  the  eruptions  were  subaqueous,  as  in  the  vicinity  of 
Dayton.  The  entire  table-land  around  that  place  is  built  up  of  stratified 
trachytic  tufa.  The  solid  trachyte  rises  from  it  in  rugged  mountains,  which 
form  an  elevated  and  very  conspicuous  range,  passing  east  of  the  inter- 
section of  Six-mile  and  Seven-mile  Canons  across  the  Seven-mile  Canon 
(where,  for  instance,  the  Sugar-Loaf  Peak  consists  of  it),  and  bending  in  a 
semicircle  round  to  "Washoe  Lake.  Farther  north  this  rock  covers  the 
country  to  a  great  extent.  Sanidin  trachyte  has  never  been  found  to  con- 
tain silver-bearing  veins,  and  in  Washoe  none  occur  in  it;  and  yet  it  has 
been  mainly  instrumental  in  the  formation  of  the  Comstock  Lode  and-  other 
veins  in  that  region.  No  geological  events  after  that  ejDOch  are  worth  men- 
tioning for  the  present  object. 

Mode  of  occurrence  of  the  Comstock. lu  1865   Only  abOUt    11,000  fcCt  of  thc  LODE 

had  been  explored  to  any  extent,  mainly  the  ground  lying  between  the 
02)hir  North  mine  and  the  Overman,  and  a  few  only  of  the  mines  had  reached 
a  depth  exceeding  700  feet.  At  an  average  depth  of  500  feet  both  walls 
were  found  dipping  to  the  east  at  from  42°  to  60°.  Above  this  level  the 
west  wall  preserved  the  same  slope,  while  the  east  wall  curved  rapidly 
toward  the  vertical,  and  then  to  the  east,  giving  the  cross-section  of  the 
vein  the  shape  of  a  funnel,  a  great  part  of  the  space  in  the  enlarged  portion 
next  the  surface  being  occupied  by  fragments  of  country  rock  or  "  horses," 
between  which  was  the  vein  matter.  The  width  of  the  belt  in  which  these 
branches  came  to  the  surface,  and  there  form  scattered  croppings,  is  gen- 
erally more  than  500  feet.  To  the  west  of  the  Lode  a  number  of  small 
veins  show  as  croppings.  They  probably  unite  with  the  Comstock  in  depth, 
and  form  with  the  latter  what  the  Germans  call  a  "Gangzug." 

The  course  of  the  west  wall,  as  far  as  explored,  is  somewhat  dependent 
on  the  shape  of  the  slope  of  the  range  at  the  base  of  which  it  lies.  It  par- 
takes of  all  its  irregularities,  passing  the  ravines  in  concave  bends,  and 
inclosing  the  foot  of  the  different  ridges  in  convex  curves;  the  greatest 
convexity  is  around  the  broad  uninterrujjted  foot  of  Movmt  Davidson  itself. 
These  irregularities  are  of  importance,  as  they  influence  the  ore-bearing 


PEEYIOUS  INVESTIGATIONS.  15 

character  of  the  vein.  The  west  wall  of  the  main  vein  is  well  defined ;  not 
so  the  east  wall,  where,  as  is  often  the  case  with  true  fissure  veins,  the  country 
rock  is  impregnated  with  matter  similar  to  that  which  fills  the  fissure.  It 
is  frequently  concentrated  in  channels  running  parallel  to  or  ascending  from 
the  vein,  but  in  fact  forming  parts  of  it. 

The  rocks  which  accompany  the  Comstock  vein  change  in  its  course. 
They  ai'e  different  varieties  of  propylite  on  the  eastern  side  throughout  its 
whole  extent.  In  some  places  the  frequent  and  large  crystals  of  feldspar 
give  it  a  porphyritic  character,  which  in  certain  varieties  is  rendered  more 
striking  by  green  columns  of  hornblende;  at  others,  the  rock  has  a  very 
fine  grain,  and  the  inclosed  crystals  are  of  minute  size;  again,  the  I'ock  is 
either  compact  and  homogeneous,  or  it  has  a  brecciated  appearance  from  the 
inclosure  of  numerous  angular  fragments;  the  color  also  changes,  though 
it  is  predominantly  green,  and  the  diff"erent  degrees  of  decomposition  create, 
finally,  an  endless  variety.  The  causes  to  which  it  is  due  will  be  consid- 
ered presently. 

The  western  country  offers  more  differences.  Along  the  slope  of 
Mount  Davidson  and  Mount  Butler,  from  the  Best  S  Belcher  mine  to 
Gold  Hill,  it  is  formed  by  syenite,  which  at  some  places  is  separated  from 
the  vein  by  a  fine-grained  and  crystalline  rock  of  black  color,  having  the 
nature  of  aphanite,  but  altogether  obscure  as  to  the  mode  of  its  occurrence. 
It  is  from  3  to  50  feet  thick,  and  the  elucidation  of  its  real  nature  may 
be  expected  from  further  developments.  As  syenite  to  the  west,  and  propy- 
lite to  the  east,  occur  just  in  that  portion  of  the  Comstock  vein  which  has 
been  most  explored,  and  where  works,  more  than  anywhere  else,  extend  in 
both  directions  into  the  country,  it  has  been  generally  assumed  in  Virginia 
that  the  Lode  follows  the  plane  of  contact  between  two  different  kinds  of 
rock,  and  is  therefore  a  contact  deposit.  But  immediately  north  of  Mount 
Davidson,  where  propylite  extends  high  up  on  the  western  hills,  this  rock 
forms  the  western  country  as  well  as  the  eastern,  as  at  the  California  and 
Ophir  mines,  though  at  the  latter  metamorphic  rocks  and  syenite  are  asso- 
ciated with  propylite  on  the  western  side.  On  Cedar  Hill  syenite  again 
predominates,  but  farther  north  propylite  forms  the  country  on  both  sides. 
South  of  Gold  Hill  the  syenite  disappears  from  the  western  wall,  and  its 


16  GEOLOGY  OF  THE  COMSTOCK  LODE. 

place  is  taken  to  some  extent  by  propylite,  but  in  greater  part  by  meta- 
morphic  rocks.  Nowhere  have  syenite  and  metamorphic  rocks  been  found 
occurring  on  the  eastern  side. 

The  formation  of  the  fissure  was  only  in  so  far  dependent  on  the  con- 
tact between  propylite  and  syenite,  as  it  follows  the  same  accidentally  in 
part  of  its  course,  probably  because  the  resistance  along  it  was  inferior  to 
that  offered  by  the  solid  masses  of  rock  on  either  side.  It  is  characteristic 
of  fissure  veins  in  general,  not  only  that  the  country  rock  on  one  side  has 
moved  downward  on  the  other,  but  also  that  within  the  space  formed  by  the 
opening  of  the  fissure  powerful  dynamic  action  has  taken  place.  Few  veins 
present  these  phenomena  so  distinctl}"  as  the  Comstock,  the  eastern  side  of 
which  has  apparently  moved  downward  on  the  western;  and  the  action 
within  the  vein  is  amply  evinced  by  the  brecciation  of  the  vein  matter,  the 
presence  of  masses  and  seams  of  clay,  the  crushed  condition  of  the  quartz,  etc. 

There  is  a  marked  difference  between  the  western  and  the  eastern  crop- 
pings.  Those  of  the  western  branches  of  the  vein  carry  principally  crys- 
tallized quartz  of  a  very  glassy  appearance,  light  color,  and  comparatively 
of  a  pure  quality.  Large  angular  fragments  -of  the  country  rock  are  em- 
bedded in  the  quartz  and  form  centers  of  its  crystallization.  Metalliferous 
minerals  are  scarce,  though  nowhere  entirely  wanting  Nothing  indicates 
underground  wealth,  nor  indeed  has  such  been  found  by  subsequent  mining. 
The  only  exception  is  Cedar  Hill,  where  native  gold  was  found  abundantly 
in  places,  but  its  scarce  distribution  never  justified  great  expectations.  In 
the  eastern  outcrop  particles  of  countr}'^  rock,  together  with  those  of  clayey 
matter  and  metallic  substances,  occur  finely  disseminated  through  the  quartz, 
which  is  reddened  by  metallic  oxides. 

Contents  of  the  Lode. — The  vcin  matter  is  composed  of  fragments  of  country 
rock,  clay,  quartz,  and  ores.  Near  the  surface  about  five-sixths  of  the  mass 
of  the  Comstock  vein  consists  of  horses,  the  shape  and  size  of  which  vary 
with  the  different  nature  of  the  rock  of  which  they  consist.  Those  of  pro- 
pylite are  confined  throughout  Virginia  City  to  the  east  side,  and  they  are, 
as  a  rule,  longer  and  thinner  than  those  of  syenite.  From  the  large  horses 
every  variety  occurs  down  to  the  smallest  fragments.  The  quartz  is  often 
so  thickly  filled  with  angular  pieces  as  to  have  a  brecciated  appearance. 


PEEVIOUS  INVESTIGATIONS.  17 

Propylite  is  much  more  common  than  syenite.  Few  large  veins  are  so  abun- 
dant in  clay  and  clayey  matter  as  the  Comstock.  It  forms  the  western  and 
eastern  selvages  from  north  to  south  in  continuous  sheets  sometimes  of  from 
10  to  20  feet  in  thickness.  Other  sheets  divide  horses  from  quartz,  or  different 
bodies  of  the  latter  from  one  another.  Most  horses  terminate  at  the  lower 
end  in  clayey  substances.  The  differences  mentioned  before  as  prevailing 
in  the  quartz  of  the  outcrops  continue  downwai'd,  but  are  not  so  conspicuous 
in  depth  on  account  of  the  general  white  color  of  the  quartz.  Finely  dis- 
seminated particles  of  wall  rock  are  always  abundant  where  the  quartz  con- 
tains ore.  The  quartz  is  generally  fractured,  and  at  numerous  places  the 
effects  of  dynamical  action  on  it  are  such  as  to  give  it  the  appearance  of 
crushed  sugar.  The  principal  silver  ores  of  the  Comstock  are  stephanite, 
vitreous  silver  ore,  native  silver,  and  very  rich  galena.  Quartz  is  the  only 
gangue,  though  carbonate  of  lime  and  gypsum  occur  in  places.  Zeolites  are 
limited  to  the  northern  portion  of  the  vein,  where  chabasite  and  stilbite  fill 
small  fissures  and  cavities  in  propylitic  breccia  within  the  body  of  the  vein. 

The  ore  is  distributed  in  a  different  way  in  the  northern  and  southern 
parts  of  the  vein.  The  passage  between  the  two  modes  of  occurrence  is 
gradual.  In  the  northern  part  the  ore  is  concentrated  in  elongated  lenticular 
masses,  of  which  the  greatest  axis  is  not  far  from  the  vertical.  The  different 
oi'e  bodies  often  adjoin  each  other  in  such  a  way  as  to  make  a  nearly  con- 
tinuous line,  as  was  the  case  in  the  Gould  &  Gurry  and  the  Savage  mines. 
The  ore  has  been  exceedingly  rich  in  the  center  of  the  different  bodies, 
where,  at  the  same  time,  it  was  soft  and  could  be  easily  removed,  while  the 
outer  parts  were  hard,  and  consisted  of  second-class  and  low-grade  ore. 

Near  the  center  of  the  Lode,  at  the  Bullion  mine,  quartz  fills  the  entire 
width  of  the  vein  from  the  western  to  the  eastern  wall,  though  it  is  too  poor 
for  extraction.  The  occurrence  of  ore  in  chimneys  and  in  barren  portions 
between  them  ceases  in  this  neighborhood.  To  the  south  the  ore  is  concen- 
trated in  continuous  sheets,  the  principal  one  of  which  is  very  near  and 
parallel  to  the  eastern  wall.  The  second  sheet  occurred  farther  to  the  west, 
extending  from  the  outcroppings  to  a  couple  of  hundred  feet  in  depth.  This 
sheet  dipped  to  the  west  at  an  angle  of  about  60°,  flattening  in  depth  and 
terminating  in  horizontal  layers  of  clay.  It  was  particularly  rich  in  gold. 
2  0  1. 


18  GEOLOGY  OF  THE  COMSTOCK  LODE. 

The  yield  of  the  ore  has  decreased  in  general  from  the  surface  down- 
ward. The  deposits  of  the  Ophir  and  Mexican  and  of  the  Gould  d  Curry 
were  the  richest.  The  former  yielded  on  an  average  $107  per  ton;  the 
latter  $70  and  $80,  notwithstanding  the  imperfect  processes  of  extraction 
which  were  formerly  applied.  Ores  of  $600  to  the  ton  were  then  no  rarity, 
and  considerable  shipments  could  be  made  of  such  as  yielded  from  $2,000 
to  $3,000  to  the  ton.  It  would  now  scarcely  be  possible  to  collect  one  ton 
of  such  ore.  The  general  average  of  all  the  ores  extracted  in  186'^  will  not 
be  more  than  $37  to  the  ton.  The  proportion  of  gold  to  silver  decreased 
during  the  early  period  of  the  working  on  the  Comstock  Lode,  but  is  now 
asrain  on  the  increase. 

Source  of  the  ore. — The  CoMSTOCK  Vein  has  neither  been  filled  from  above 
nor  from  the  sides,  as  none  of  the  surrounding  rocks  could  have  yielded  the 
immense  quantity  of  vein  matter  and  ore;  and  had  it  been  formed  in  this 
way,  the  mass  would  have  a  banded  and  comby  structure,  which  is  by  no 
means  observable.  The  eastern  rock  may,  on  account  of  its  extensive 
decomposition,  appear  to  favor  the  assumption  of  lateral  infiltration;  but 
this  decomposition  was  effected  by  ascending  currents,  which  have  left 
distinct  traces,  and  which  could  not  have  removed  any  matter  in  a  lateral 
way.  Thermal  springs,  which  are  considered  by  many  authorities  as  the 
asrent  which  carried  mineral  matter  from  below  into  fissures  and  to  have 
formed  every  true  vein,  would  not  explain  the  formation  of  the  Comstock 
Lode.  Silica,  in  such  cases,  is  accumulated  round  the  mouth  of  the  fissures, 
and  though  ordinarily  removed  by  denudation,  it  could  hardly  be  supposed 
to  be  so  at  the  Comstock  vein,  as  since  its  formation  the  sui-face  has  under- 
gone but  slight  changes.  But,  besides,  the  decomposition  of  the  eastern 
country  for  miles  in  extent  cannot  be  explained  by  the  action  of  thermal 
springs. 

The  Comstock  fissure  is,  of  course,  of  more  recent  origin  than  the 
rocks  which  it  traverses;  and  as  propylite  is  predominant  in  the  latter,  the 
fissure  must  necessarily  have  succeeded  it  in  age.  The  only  event  after 
the  outbursts  of  propylite  capable  of  producing  such  powerful  action  was 
the  eruption  of  trachyte,  which  accomjDanies  the  vein  at  a  distance  of  two 
miles  to  the  east.     As  there  is  other  evidence  of  its  intimate  connection  with 


PREVIOUS  LNVESTIGATIONS.  19 

the  CoMSTOCK  vein,  we  may  take  it  for  granted  that  it  caused  the  rending  of 
the  fissure. 

Applicability  of  the  ascension-theory. — We  have  iu  the  elcments  evolved  during 
the  first  two  periods  of  solfataras,  namely,  fluoiine,  chlorine,  and  sulphur, 
all  the  conditions  required  for  filling  the  CoMhTOCK  fissure  with  such  sub- 
stances as  those  of  which  the  vein  is  composed.  Steam,  ascending  with 
vapors  of  fluosilicic  acid,  created  in  its  upper  parts  (by  diminution  of  pressure 
and  temperature,  according-  to  well-known  chemical  agencies)  silica  and  sili- 
cofluohydric  acid,  the  former  in  solid  form,  the  latter  as  a  volatile  gas,  which 
acts  most  powerfully  in  decomposing  the  rocks  it  meets  on  its  course.  The 
chloride  of  silicon  in  combination  with  steam  forms  silica  and  chlorhydric 
acid  Fluorine  and  chlorine  are  the  most  powerful  volatilizers  known,  and 
form  volatile  combinations  with  almost  every  substance.  Besides  silicon, 
the  metals  have  a  great  affinity  with  them.  All  those  which  occur  in  the 
CoMSTOCK  vein  could  ascend  in  a  gaseous  state  in  combination  with  one  or 
the  other  of  them.  They  must  then  be  precipitated  in  the  upper  parts  as 
metallic  oxides  or  chlorides,  and  in  the  native  state.  Thus  the  fissure  was 
gradually  filled  from  its  upper  portion  downwards,  with  all  the  elements 
which  we  find  chemically  deposited  in  it.  A  fissure  is  ordinarily  not  sta- 
tionary after  its  first  opening;  but  by  subsequent  action  may  from  time  to 
time  widen  and  frequently  contract  again.  New  channels  would  thus  be 
opened  where  the  old  ones  were  obstructed.  If  such  widening  or  opening  of 
an  empty  space  within  the  matter  filling  the  old  fissui-e  was  followed  by 
emanations  rich  in  metallic  vapors,  then  the  conditions  would  be  given  for 
the  formation  of  a  body  of  ore  of  the  shape  of  the  newly-opened  chasm, 
which  corresponds  precisely  to  most  of  the  bodies  of  ore  in  the  Comstock 
Lode.  Contemporaneously  with  the  filling  of  the  fissure,  the  adjoining  rock 
would  be  acted  upon  by  the  ascending  acid  vapors,  and  its  nature  by  them 
entirely  changed.  Cracks  would  form  in  it,  and  be  filled  with  substances 
similar  to  those  of  the  vein  itself  As  the  Comstock  vein  has  an  eastern 
dip,  and  the  action  of  forces  manifests  itself  towards  the  surface,  only  the 
rock  on  the  hanging  wall,  or  the  eastern  country,  would  be  influenced  in 
this  way.  Crevices  branching  off  from  the  main  fissure  would  probably  pen- 
etrate into  the  hanging  wall,  and  it  may  reasonably  be  expected  that  deeper 


20  GEOLOGY  OF  THE  COMSTOOK  LODE. 

workings  will  disclose  such  branches  filled  with  vein  matter,  and  probably 
of  oi'e-bearing  character,  east  of  the  main  body  of  the  vein. 

Alteration  of  minerals  in  situ. — A  transforming  actlon  must  necessarily  take 
place  from  the  very  commencement  of  the  decomposition  of  matter  in  the 
fissure.  Svilphurous  acid  and  sulphuretted  hydrogen,  which  were  among  the 
escaping  gases  in  the  first  period,  together  with  combinations  of  fluorine 
and  chlorine,  gradually  became  predominant,  marking  the  second  period  of 
solfataric  action.  But  little  more  matter  could  be  introduced  into  the  fissure, 
as  the  combinations  of  sulphur  with  mineral  substances  are  not  volatile. 
Chemical  transformation  was  now  the  principal  action  within  the  vein.  Silica 
is  deposited  from  its  combinations  with  fluorine  and  chlorine  in  a  gelatinous 
state,  very  different  in  its  physical  character  from  those  of  the  crystalline 
quartz  which  fills  the  vein.  It  must  undergo  a  solution  in  water,  with  which, 
in  the  form  of  steam,  it  was  impregnated,  in  order  to  assume  this  character. 
Metallic  oxides  and  chlorides  were  converted  into  sulphurets,  and  the  pres- 
ence of  antimony  caused  the  formation  of  sulph-antimoniurets,  the  principal 
one  of  which  is  stephanite.  By  such  processes  the  entire  vein  matter  was 
gradually  converted  from  its  former  condition  into  that  which  it  presents  at 
this  day. 

Fluorine  and  chlorine. — It  is  a  fact  worthy  of  notico  that  there  is  scarcely  a 
single  chemical  agent,  excepting  fluorine  and  chlorine,  which  would  not 
carry  metallic  substances  into  fissures  in  exactly  or  nearly  the  reverse  quan- 
titative proportion  from  that  in  which  they  occur  in  silver  veins.  Iron  and 
manganese  are  not  only  more  abundant  in  rocks,  but  also  much  more  easily 
attacked*  and  carried  away  by  acids  than  silver  and  gold.  The  proportion 
of  these  to  the  former  ought,  therefore,  to  be  still  smaller  in  mineral  veins 
than  it  is  in  rocks,  and  lead  and  copper  ought  to  be  more  subordinate,  if 
their  removal  from  their  primitive  place  had  been  effected  by  other  agents 
than  fluorine  and  chlorine.  Only  these  two  will  first  combine  with  those 
metals  which  are  most  scarce  in  rocks,  and  relatively  most  abundant  in  sil- 
ver veins;  and  they  are  probably  the  only  elements  which  have  originally 
collected  them  together  into  larger  deposits,  though  these  may  subsequently 
have  undergone  considerable  changes,  and  water  may  have  played  altogether 
the  most  prominent  part  in  bringing  them  into  their  present  shape. 


PEEVIOUS  INTESTIGATIONS.  21 

Wide-spread  soifataric  action. — Thougli  it  seems  that  the  CoMSTOCK  fissui'e  was 
the  principal  theater  for  the  emission  of  steam,  and  all  those  phenomena 
which  may  be  comprised  by  the  name  of  soifataric  action,  yet  the  latter  left 
its  traces  over  a  wide  extent  of  the  adjacent  country.  The  entire  belt  of 
rounded  hills,  extending  east  of  the  vein  for  two  miles,  to  the  foot  of  the 
trachytic  range,  shows  its  effects  very  conspicviously.  It  consists  of  propy- 
lite,  which,  however,  can  scarcely  be  recognized  on  account  of  the  complete 
decomposition  it  has  undergone,  and  which  has  transformed  it  into  a  clayey 
rock  of  red  and  yellow  color,  but  still  showing  distinctly  the  inclosed  crystals 
of  feldspar  and  hornblende.  It  is  traversed  by  numerous  crevices  from 
which  the  decomposition  originated,  and  shows  everywhere  evidences  of 
vertically  ascending  currents  which  caused  it.  Whoever  has  seen  active 
solfataras  will  be  struck  by  the  resemblance  of  chemical  action  on  the  sur- 
rounding rocks  to  that  displayed  in  the  region  east  of  the  Comstock  Lode. 
Near  some  of  the  crevices  the  decomposed  rock  is  strongly  impregnated  with 
silica,  producing  the  ranges  of  red-colored  bluffs  which  accompany  the 
Comstock  vein  to  the  east,  and  which  have  been  partly  located  as  out- 
croppings  of  veins,  while  at  about  two  miles'  distance  real  metalliferous 
veins  occur,  promising  in  their  outcrops,  but  not  yet  explored.  Besides 
this  belt  the  former  action  of  solfataras  is  plainly  visible  in  many  parts  of 
the  country.  The  formation  of  the  Comstock  vein  is  but  one  of  its  mani- 
festations. 

Continuity  of  the  Lode  in  depth. — As  it  has  bccu  shown  that  the  vciu  was  filled 
from  a  deep-seated  source,  it  is  certain  that  it  is  continuous  in  depth.  The 
inclination  is  not  likely  to  vary  considerably,  for  not  only  is  the  west  wall 
remarkably  regular,  tending  to  show  its  continuity,  but  the  previously  men- 
tioned soifataric  action  to  the  east  is  an  evidence  that  the  vein  underlies  the 
country  rock  in  this  direction  for  a  long  distance.  As  for  the  mean  width  of 
the  vein  in  depth  no  definite  prediction  can  be  made.  In  some  places  the  vein 
at  a  distance  of  500  feet  from  the  sm-face  forms  a  channel  of  120  feet  or  more 
in  width;  at  other  points  it  is  contracted.  Such  places  must  necessarily  occur 
in  an  inclined  vein  of  some  magnitude,  since  the  hanging  wall,  during  the 
long  periods  of  the  filling  of  the  fissure,  required  some  support.  The  walls  of 
every  true  fissure  vein  are  uneven  planes.     The  downward  movement  of  one 


22  GEOLOGY  OF  THE  COMSTOCK  LODE. 

side  of  the  fissure  on  the  other,  at  the  time  of  the  formation  of  the  vein, 
caused  pi'otuberances  of  one  wall  to  meet  such  of  the  other,  and  concave 
places  to  come  opposite  to  each  other.  This  is  the  reason  why  every  large 
fissure  vein  is  liable  to  repeated  expansions  and  contractions,  .though  the 
former  prevail  largely  over  the  latter.  It  is  to  be  expected  that  the  Com- 
STOCK  Lode  will  exhibit  the  same  feature  in  its  downward  course  to  indefi- 
nite depth,  as  it  has  done  heretofore,  though  its  genei-al  width  will  probably 
remain  nearly  equal  to  that  which  it  possesses  in  the  lowest  works.  The 
formation  of  large  horses  is,  from  the  nature  of  their  origin,  more  peculiar 
to  upper  than  to  lower  levels,  since  their. breaking  down  from  the  hanging 
wall  will  in  every  fissure  be  most  apt  to  take  place  where  the  latter  is  of 
comparatively  inferior  thickness,  than  where  it  is  hundreds  or  thousands  of 
feet  wide.  But  small  fragments  may  separate  from  it  at  any  depth,  and 
their  quantity  will  chiefly  depend  upon  the  nature  of  the  rock  and  the  power 
of  decomposing  agents.  If  any  change  as  to  the  inclosing  rocks  should 
occur  in  depth,  it  is  probable  that  propyhte  will  disappear  on  the  western 
side  and  syenite  predominate  there  more  and  more. 

Probable  character  of  the  Lode  in  depth. All     thc      CvideriCBS     iu     tllC     UppCr     IcVcls 

justify  the  expectation  that  the  foot  wall  will  continue  with  its  smooth  and 
regular  clay-selvage,  while  the  irregularity  and  indistinctness  of  the  eastern 
side  will  not  diminish  but  rather  increase  as  its  true  character  as  hanging 
wall  will  become  more  conspicuous.  The  vertical  sheets  of  clay  which 
have  from  time  to  time  been  cut  in  the  adits  east  of  the  vein,  rise  undoubt- 
edly from  the  hanging  wall.  Clay  seams  within  the  body  of  the  vein  will 
probably  diminish  with  the  increase  of  unity  in  inclination.  Those  which 
are  at  present  observable  at  upper  levels  are  particularly  occasioned  by 
the  vertical  position  of  the  vein-matter,  which  of  course  facilitates  sliding 
motions.     Larger  accumulations  of  clay  will  especially  continue  near  the 

old  ravines. 

The  ores,  through  all  the  levels  explored,  retain  the  character  of  true 
silver  ores  which  they  had  near  the  surface.  The  amount  of  lead,  copper, 
iron,  and  zinc  has  never  been  large  in  the  Comstock  ores,  and  these  metals 
preserve  now  at  the  lowest  levels  nearly  the  same  relative  proportion  as 
formerly.     Their  increase,  especially  of  lead,  would  be  the  most  unfavora- 


PEEVIOCJS  nsrVESTIGATlONS.  23 

ble  indication  for  the  future  of  the  Comstock  Lode  ;  as,  besides  the  growing 
difficulty  of  metallurgical  treatment,  the  conclusion  would  be  justified  that 
lead  ores  would  more  and  more  replace  those  of  silver,  and  the  limits  of 
profitable  productiveness  would  soon  be  reached.  But  as  it  is,  no  deterio- 
ration is  to  be  expected,  even  if  an  impoverishment  takes  place.  It  thus 
approaches  in  its  ore-bearing  character  the  great  mother-veins  of  Mexico, 
and  is  different  from  those  of  Hungary. 

Conclusions. — Considenug  these  facts  exhibited  by  the  Comstock  vein 
itself,  and  comparing  them  with  what  is  known  about  similar  argentif- 
erous veins,  we  believe  we  are  justified  in  drawing  the  following  conclu- 
sions : 

1st.  That  the  continuity  of  the  ore-bearing  character  of  the  Comstock 
Lode  in  depth  must,  notwithstanding  local  interruptions,  be  assumed  as  a 
fact  of  equal  certainty  with  the  continuity  of  the  vein  itself. 

2d.  That  it  may  be  positively  assumed  that  the  ores  in  the  Comstock 
Lode  will  retain  their  character  of  true  silver  ores  to  indefinite  depth. 

3d.  That  it  is  highly  probable  that  extensive  bodies  of  ore  equal  in 
richness  to  the  surface-bonanzas  will  never  recur  in  depth. 

4th.  That  an  increase  in  size  of  the  bodies  of  ore  in  depth  is  more 
probable  than  a  decrease,  and  that  they  are  more  likely  to  increase  than 
to  remain  of  the  same  size  as  heretofore. 

5th.  That  a  considerable  portion  of  the  ore  will,  as  to  its  yield,  not  mate- 
rially differ  at  any  depth  from  what  it  is  at  the  present  lower  levels ;  while 
besides  there  will  be  an  increasing  bulk  of  low-grade  ores.  We  are  led  to 
this  supposition  by  the  similarity  in  character  of  all  the  deposits  outside  of 
the  rich  surface-bonanzas,  and  the  homogeneous  nature  which  almost  every 
one  of  them  exhibits  throughout  its  entire  extent. 

6th.  That  the  ore  will  shift  at  different  levels  from  .certain  portions  of 
the  Lode  to  others,  as  it  has  done  up  to  the  present  time.  More  equality 
in  its  distribution  may,  however,  be  expected  below  the  junction  of  the 
branches  radiating  toward  the  surface,  when  the  vein  will  probably  fill  a 
more  uniform  and  more  regular  channel.  Some  mines  which  have  been 
heretofore  almost  unproductive,  as  the  Central,  California,  Bullion,  and  others, 
have  therefore  good  chances  of  becoming  metalliferous  in  depth.     But 


24  GEOLOGY  OF  THE  COMSTOCK  LODE. 

throughout  the  extent  of  the  vein  it  is  most  likely  that  the  portion  which 
lies  next  to  the  foot  wall  will  continue  unproductive,  as  it  did  from  the  sur- 
face down  to  the  lowest  works,  while  the  entire  portion  between  it  and  the 
hanging  wall  must  be  considered  as  the  probable  future  source  of  ore.  As 
remarked  in  the  foregoing  pages,  it  is  also  probable  that  repeatedly,  in  fol- 
lowing the  Lode  downward,  branches  will  be  found  rising  from  its  main 
body  vertically  into  the  hanging  wall,  and  consisting  of  clay  and  quartz. 
Many  of  them  will  probably-be  ore-bearing.  Such  bodies  of  ore  should  be 
sought  for  at  all  mines,  in  what  is  generally  supposed  to  be  the  eastern 
country.  Experience  in  the  upper  levels  would  lead  to  the  supposition  that 
such  eastern  bodies  might  carry  richer  ores  than  the  average  of  the  main 
portion  of  the  vein. 

7th.  That  the  intervention  of  a  barren  zone,  as  is  reported  by  good 
authorities  to  occur  at  the  Veta  Madre  of  Guanajuato,  at  the  depth  of  1,200 
feet,  is  not  at  all  likely  to  be  met  with  in  the  case  of  the  Comstock  Lode. 
The  argument  which  we  have  to  adduce  for  this  conclusion  has  some  weight 
from  a  geological  point  of  view.  It  is  a  well-known  fact  that  the  inclosing 
rocks  have  usually  great  influence  on  the  quantity  and  quality  of  the  ores 
of  cei'tain  metals  in  mineral  veins,  and  that  a  rich  lode  passing  into  a  differ- 
ent formation  frequently  becomes  barren  or  poor.  At  the  Veta  Madre  of 
Guanajuato  a  sudden  decrease  in  the  yield  of  the  ore,  at  the  depth  of  1,200 
feet,  attends  the  passage  of  the  lode  into  a  different  formation,  which  from 
thence  continues  to  the  lowest  depth  attained.  No  such  change  can  ever 
be  anticipated  for  the  Comstock  Lode,  since  the  structure  of  the  country 
seems  to  indicate  the  continuity  of  the  inclosing  rocks  to  an  indefinite 
depth. 

King's  memoir. — In  1867-'68  Mr.  Clareuce  King,  Geologist-in-charge  of 
the  Exploration  of  the  Fortieth  Parallel,  made  an  examination  of  the  Com- 
stock LoDF.^  In  regard  to  lithology  Mr.  King  mainly  followed  Baron  v. 
Richthofen,  but  he  recognized  a  much  greater  area  of  andesite  than  his 
predecessor,  and  the  affinity  between  certain  propylites  and  andesites.  Of 
the  latter  he  says:  "The  balance  of  probability  points  to  a  close  alliance 

1  Exploration  of  the  Fortieth  Parallel,  Vol.  III. 


PEEVIOUS  INVESTIGATIONS.  25 

between  this  rock  and  propylite,  and  it  will  not  be  at  all  surprising  if  it 
should  finally  prove  to  be  chemically  identical  and,  in  reality,  only  a  different 
form."  The  rock  determined  as  quartz-porphyry  by  Baron  v.  Richthofen, 
Mr.  King  regarded  as  quartz-propylite. 

In  discussing  the  relation  of  Mount  Davidson  to  the  vein,  Mr.  King 
calls  especial  attention  to  the  agreement  between  the  contours  of  the  west 
wall  of  the  Lode  and  those  of  the  exposed  face  of  the  range.  He  considers 
this  similarity  as  evidence  that  the  wall  is  merely  a  continuation  of  the  face 
of  Mount  Davidson,  and  that  the  syenite  has  undergone  very  little  erosion 
since  the  opening  of  the  fissure.  It  is  not  necessary  to  summarize  Mr. 
King's  very  graphic  description  of  the  structure  of  the  Comstocic.  Lode  in 
detail  here,  as  I  shall  be  obliged  to  use  it  in  my  own  account  of  the  vein, 
the  portion  which  he  examined  having  long  been  inaccessible. 
Mr.  King  closes  his  memoir  with  the  following  summary : 
King's  summary. — "  Tlic  ancicut  Virginia  Range,  prior  to  the  Tertiary  period, 
was  composed  of  sedimentary  beds  of  the  great  Cordillera  system,  which, 
in  the  late  Jurassic  epoch,  had  been  folded  up,  forming  one  of  the  corruga- 
tions of  that  immense  mountain  structure  which  covers  the  western  front  of 
our  continent.  Accompanying  this  upheaval  were  outpourings  of  granite 
and  syenite.  The  erosion  which  followed  this  mountain  period  escarped 
the  ancient  rocks,  and  modeled  the  eastern  front  of  Mount  Davidson  into  a 
comparatively  smooth  surface,  whose  average  angle  of  slope  sank  to  the 
east  at  about  40°.  In  the  late  Tertiary,  at  the  time  of  the  volcanic  era,  the 
Virginia  Range  shared  in  the  dynamical  convulsions  which  gave  vent  to  suc- 
cessive volcanic  outflows  of  immense  volume  and  very  remarkable  character. 
The  first  and,  so  far  as  the  Comstock  Lode  is  concerned,  the  most  import- 
ant was  of  propylite,  or  trachytic  greenstone,  which  deluged  the  range  from 
summit  to  base,  covering  large  portions  of  its  ancient  surface,  and  leaving 
here  and  there  isolated  masses,  which  rose  like  islands  above  the  wide 
fields  of  volcanic  rock.  Subsequently  followed  the  period  of  the  andesites 
which,  at  their  commencement,  in  the  form  of  a  thin  intrusive  dike,  pene- 
trated a  new-formed  fissure  on  the  contact  plane  of  the  ancient  syenite  and 
the  propylite.  This  earlier  andesite  period  gave  birth  to  the  solfataras, 
which,  bursting  from  a  hundred  vents,  rapidly  decomposed  the  surrounding 


26  GEOLOGY  OF  THE  COMSTOCK  LODE. 

rocks,  and  gradually  filled  the  fissures  of  the  Comstock  with  their  remarkable 
charges  of  metal-bearing  quartz.  The  latest  flows  of  andesite  poured  out 
over  the  decomposed  propylite;  and  since  they  are  themselves  unaltered, 
their  appearance  marks  the  period  when  solfataric  action  over  wide  areas 
had  ceased.  While  it  no  longer  maintained  its  energy  through  the  broad 
zone  of  propylite,  it  still  continued  intensely  active  within  the  chambers  of 
the  Comstock  Lode.  Metallic  contents  were  introduced  into  the  quartz, 
the  clay  seams  were  formed  by  a  rapid  decomposition  of  the  neighboring 
propylite  materials,  the  horses  reduced  to  a  spongy,  semi-plastic  condition, 
and  at  last  the  final  solidification  of  the  quartz  took  place.  Outside  the 
vein  two  events  of  geological  interest  have  occurred:  first,  the  period  of 
trachyte  eruptions,  when  from  the  ruptures  of  the  crust,  parallel  to  the 
Comstock  Lode,  vast  volumes  of  sanidin-trachyte  overflowed  the  country; 
and,  secondly,  the  less  powerful  but  still  important  outpouring  of  basaltic 
rock,  which  marked  the  close  of  the  volcanic  era.  Within  the  vein,  and 
probably  caused  by  one  or  both  of  these  latter  volcanic  disturbances,  a 
pressure  has  been  exerted  which  has  crushed  and  ground  the  masses  of 
quartz  into  minute  fragments.  It  is  interesting  to  observe  that  while  this 
force  was  great  enough  to  crush  quartz  masses  1 50  feet  in  breadth  into  mere 
angular  pebbles,  the  disturbances  were  insufficient  to  cause  any  actual 
faulting  of  importance.  Both  within  and  without  the  vein  the  solfataras 
gradually  came  to  a  close.  The  heated  currents  of  water  which  even  yet 
ascend  into  the  lower  levels  of  the  mines,  are  evidence  that  at  no  very 
great  depth  a  considerable  temperature  is  still  maintained;  but  this  is  only 
a  faint  relic  of  a  once  intense  action." 

zirkei's report. — lu  ISTH  Prof.  Ferdiuaud  Zirkel  made  a  macroscopical  and 
mici'oscopical  examination  of  the  lithological  collections  of  the  Exploration 
of  the  Fortieth  Parallel.^  Among  the  slides  which  he  described  are  thirty- 
three  from  the  Washoe  district  He  confirmed  the  independence  of  horn- 
blende-propylite  and  quartz-propylite  as  lithological  species,  regarded  most 
of  the  quartzose  rock  as  dacite,  coiTCCted  the  determination  of  the  granu- 
lar diorite  (which  had  been  considered  as  syenite),  and  added  augite-ande- 

'  Exploration  of  the  Fortieth  Parallel,  Vol.  VI. 


PREVIOUS  INVESTIGATIOlSrS.  27 

site,  rhyolite,  and  a  strange  variety  of  basalt  to  the  list  of  rocks  previously 
recognized.  Professor  Zirkel  formulates  the  diagnostic  differences  between 
propylite  and  andesite  as  follows:-^ 

propyiite. — "o  The  general  color  of  the  propylitic  groundmass  has  more 
of  a  greenish-gray,  while  the  andesitic  groundmass  has  more  of  a  pure  gray 
or  brown  tinge. 

"b.  In  structure  and  in  the  behavior  of  its  constituents,  the  propylite 
still  resembles  the  older  ante-Tertiary  diorite-porphyries. 

"c.  The  groundmass  of  the  propylites  is  rich  in  minute  particles  of 
hornblende,  while  in  that  of  the  andesites  this  mineral  appears  only  in  the 
larger  individuals,  fine  hornblende  dust  being  wanting. 

"d  The  propylitic  feldspars  are  usually  filled  with  a  considerable 
quantity  of  hornblende  dust,  while  the  andesitic  feldspars  are  entirely  with- 
out it:  the  latter  not  infrequently  containing  glass-inclusions,  which  do  not 
seem  to  occur  in  the  propylitic  plagioclases. 

"e.  The  color  of  the  proper  hornblende  sections  in  propylite  is  always 
green  (never  brown),  while  the  color  of  those  in  andesites  is  almost  with- 
out exception  brown  ;  and  the  propylitic  hornblende  never  shows  the  curious 
black  border  which  is  so  common  to  that  of  andesites;  and  again,  propylite 
in  some  cases  contains,  besides  the  largely  predominating  green  hornblende, 
a  few  sections  of  the  brown  mineral,  presenting,  in  many  points,  a  strikingly 
peculiar  aspect,  while  in  andesites  two  kinds  of  hornblende  never  occur 
together. 

"/  The  propylitic  hornblende  is  often  distinctly  built  up  of  thin 
needles  or  staff-like  microlites,  and  therefore  is  not  regularly  cleavable ; 
which  has  never  been  found  to  be  the  case  in  andesites. 

"(jr.  The  production  of  microscopical  epidote  (mainly  by  the  alteration 
of  hornblende),  so  very  common  in  propylites,  has,  with  one  exception, 
never  been  observed  in  these  andesites,  and  it  is  also  unknown  in  the  Euro- 
pean occurrences. 

"h.  Augite  often  occurs  as  an  accessory  constituent  in  andesites,  but 
it  is  comparatively  very  rare  in  propylites. 

"i.  The  andesitic  groundmass  here  and  there  seems  to  possess  a  half- 
glassy  development:  a  glass-bearing  propylitic  groundmass  has  never  been 

'  Exploration  of  the  Fortieth  Parallel,  Vol.  VI.,  p.  132. 


28  GEOLOGY  OF  THE  COMSTOCK  LODE. 

found;  and  herein  is  another  point  of  resemblance  to  the  old  diorite-por- 
phyries. 

"All  these  differences  between  propylitic  and  andesitic  hornblende  also 
extend  to  both  of  the  quartziferous  members,  quartz-propylite  and  dacite." 

On  page  1 1 7  Professor  Zirkel  states  that  the  quartz  of  quartz-propylite 
contains  fluid  inclusions,  and  "behaves  exactly  like  that  of  the  ante-Tertiary 
dioritic  porphyries,  and  differently  from  that  of  all  other  Tertiary  quartz- 
iferous rocks,  dacites  and  rhyolites,  which  only  contain  glass  inclusions." 

Church's  memoir. — lu  1877  Mr.  J.  A.  Church  made  an  examination  of  the 
CoMSTOCK,^  as  a  member  of  the  United  States  Surveys  West  of  the  One 
Hundredth  Meridian,  under  Captain  Wheeler.  Mr.  Church  accepted  the 
lithology  of  his  predecessors  with  some  modifications  a  little  difficult  to  fol- 
low, but  thoucrh  he  mentions  slides  of  the  rocks,  describes  none.  His 
memoir  contains  a  number  of  ingenious  hypotheses  which  would  possess 
great  importance  if  sufficient  evidence  in  their  favor  could  be  adduced. 

Lithology. — Mr.  Church  appears  to^use  the  terms  porphyrite  and  propylite 
interchangeably  for  all  strikingly  porphyritic  rocks  of  light  color.^  Rocks 
of  dark  color,  whether  from  the  presence  of  abundant  hornblende  or  from  the 
transparency  of  the  feldspars,  he  appears  to  have  regarded  as  andesite,'  and 
asserts  that  it  is  quite  safe  to  put  the  minimum  number  of  north  and  south 
dikes  of  this  rock  at  between  twenty-five  and  fifty.  Besides  the  masses 
which  had  hitherto  been  regarded  as  trachyte,  he  determined  the  rocks 
about  the  new  Yellow  Jacket  shaft,  and  at  other  points,  as  remnants  of  the 
trachyte  eruption.  This  leads  to  the  supposition  that  he  employed  the  term 
merely  to  designate  soft,  rough,  light-colored,  porphyritic  rocks.  In  Mr. 
Church's  opinion  the  diorite,  propylite,  and  probably  the  andesite,  were  laid 
down  in  thin  regular  layers,  which  he  compares  to  those  of  sedimentary 
rocks.  This  he  considers  proved  by  the  sheeted  character  of  the  rocks  in 
the  east  and  west  country. 

'The  CoMSTOCK  Lode,  its  formation  and  history.  By  John  A.  Church,  E.  M.,  Ph.  D.,  member 
of  the  American  Institute  of  Mining  Engineers,  mining  engineer.  Illustrated  by  six  plates  and  thir- 
teen figures.     New  York :  John  Wiley  &  Sons.     1879. 

=  L.  c.,  p.  40  to  42  and  52. 

'Ii.  c,  pp.  37  and  47.     The  McKibben  tunnel  shows  only  diorites  and  quartz. 


PEEVIOUS  INVESTIGATIONS.  29 

History  of  the  lode. — Mr.  Churcli  dividcs  the  history  of  the  Comstock  Lode 
into  nine  epochs  :^ 

1.  The  diorite  epoch. — The  horizontal  deposition  of  diorite,  which  is  one 
of  the  fine-grained,  thin-running  lavas,^  in  stratified  layers,  by  a  series  of 
eruptions. 

2.  The  subordinate  pressure. — The  system  of  diorite  strata  was  acted 
upon  by  a  pressure  which  produced  broad  folds  with  east  and  west  axes, 
an  uphft  in  Virginia,  and  a  trough  in  Gold  Hill.  This  important  force 
continued  to  affect  the  rocks  through  the  greater  part  of  their  history, 
and  is  the  dynamic  cause  of  the  Lode. 

3.  The  propylite  epoch.— The  horizontal  deposition  of  the  propylite,  also 
in  stratified  layers  from  successive  fissures.  The  members  of  the  new  rock 
are  essentially  parallel  to  the  older  layers. 

4.  The  principal  elevation. — After  the  propylite,  came  a  movement  by 
which  the  two  series  of  eruptive  depositions  were  raised  into  a  mountain 
system.  This  elevation  took  place  about  a  north  and  south  axis,  and  its 
folds  are  therefore  at  right  angles  to  those  of  the  former  movement. 

5.  The  andesite  epoch. — A  third  period  of  eruption  follows,  the  seat  of 
which  is  the  upturned  strata  of  the  diorite  and  propylite.  These  are  not 
fractured  except  near  the  eroded  surface,  but  the  layers  are  separated,  and 
the  andesite  rises  through  the  crevices,  establishing  an  extensive  system  of 
bedded  dikes.  The  whole  mass  of  erupted  andesite  is  assumed  to  have 
been  some  thousands  of  feet  thick,  and  to  have  played  an  important  part  in 
the  history  of  the  Lode  by  its  weight  and  rigidity. 

6.  The  opening  of  the  strata — The  crests  of  folds  already  produced  were 
lifted  forcibly  against  the  rigid  andesite  cap,  while  the  intervening  troughs 
were  bent  downward,  relieving  them  from  its  weight.  Under  this  action 
the  uplifted  portions  of  the  strata  were  squeezed  sidewise  into  the  relieved 
troughs,  opening  slightly  the  partings  between  the  layers. 

7.  The  silicious  epoch. — Through  the  small  partings  of  the  strata  thus 
opened,  rose  currents  of  water  holding  silica  in  solution.  The  strata  sub- 
jected to  their  action  were  dissolved  or  carried  off  mechanically,  and  quartz 
with  "base"  metals  was  deposited  in  their  place.    This  action  went  on  in  each 

'L.  c,  p.  128.  «L.  c,  p.  153. 


30  GEOLOGY  OF  THE  COMSTOCK  LODE. 

of  the  open  seams,  the  intervening  rock  being  attacked  from  both  sides 
until  the  meeting  of  several  depositions  of  silica  composed  quartz  bodies, 
which  in  many  cases  had  a  thickness  of  several  hundred  feet.  This  quartz 
was  not  argentiferous,  and  no  ore  was  formed.  A  second  important  result 
of  this  appearance  of  silicious  waters  is  the  almost  entire  removal  of  the 
immense  andesite  cap,  which  was  decomposed  in  the  same  manner  as  the 
deeper-lying  rocks. 

8.  The  trachyte  epoch. — New  crevices  opened  in  the  eastern  part  of  the 
district,  and  vast  floods  of  trachyte  poured  out.  Instead  of  resisting  move- 
ment hke  the  andesite,  it  loaded  down  the  hanging  wall  of  the  Lode  so 
heavily  that  it  slid  upon  the  foot  wall.  This  action  resulted  in  an  entirely 
new  system  of  openings.  Near  the  surface  the  new  crevices  abandoned  the 
old  line  of  quartz  deposition,  and  broke  through  the  hanging  wall  in  a  more 
or  less  nearly  vertical  direction. 

9.  The  argentiferous  epoch. — Into  these  new  crevices  poured  a  second 
stream  of  water  containing  minerals  in  solution,  but  differing  from  the  first 
in  holding  not  only  silica,  but  also  silver  and  gold. 

"The  facts  here  brought  forward,"  says  Mr.  Church,  "show  that  no 
vein  and  nothing  like  a  real  vein  exists  in  ground  that  has  for  years  been 
supposed  to  contain  the  boldest  example  of  true  fissure  vein  formatioh  in 
the  world;  that  the  largest  bodies  of  ore  can  be  formed  from  deep  sources 
of  mineral  supply  without  the  agency  of  a  fracture  even  of  the  smallest 
dimensions;  and  that  it  is  quite  unnecessary  to  seek  for  great  dynamical 
convulsions  to  account  for  the  formation  of  thick  masses  of  ore  within  the 
solid  rock,  a  sufficient  cause  being  found  in  the  quiet  action  of  the  same 
forces  whicli  have  everywhere  molded  the  crust  of  the  earth." 

The  Justice  ore  body  Mr.  Church  regards  as  a  deposit  wholly  distinct 
from  the  Comstock,  though  attributable  to  the  same  general  causes,  and  as 
formed  in  a  similar  way. 

Pi,y3i,3._0f  the  finely-divided  quartz  known  as  sugar  quartz,  he  says  ■} 
"The  grains  are  remarkable  in  never  being  crystalline,  the  microscope  not 
reveahng  one  crystal  in  millions  of  particles."  And  again i^  "The  lesson  to 
be  derived  from  the  sugar  quartz  is  not  that  it  has  been  crushed,  but  that  it  has 

iL.  c,  p.  85.  »L.  0.,  p.  151. 


PEEVIOUS  INVESTIGATIONS.  31 

been  preserved  from  crushing.  It  was  formed  in  the  state  of  powder,  and 
since  its  deposition  the  Lode  rocks  have  not  received  any  addition  which 
could  weigh  it  down.  On  the  other  hand,  the  barren  quartz  was  probably 
laid  down  in  a  similar  state  of  powder,  and  has  been  consolidated  by  the 
load  of  trachyte  upon  the  surface." 

The  heat  of  the  Lode  Mr.  Church  ascribes  to  the  kaolinization  of  feld- 
spar, supporting  this  view  by  the  statement,  that  as  kaolinization  involves 
hydration,  heat  must  be  liberated,  and  by  the  assertion  that  flooded  drifts 
grow  hotter.  He  believes  the  heat  to  be  diffused  by  hot  aqueous  vapor 
permeating  the  rocks.     The  latter,  he  asserts,  are  in  large  part  perfectly  dry. 

Technical  literature. — Thougli  most  of  tlic  scieutlfic  aud  tcchuical  journals 
contain  papers  on  the  Comstock,  or  items  referring  to  it,  and  much  space  is 
occupied  by  the  same  subject  in  the  reports  of  the  United  States  Mining 
Commissioners  and  of  the  State  Mineralogist  of  Nevada,  I  am  not  aware 
of  any  further  noteworthy  contributions  to  its  geology.  The  numerous 
geological  suggestions  thrown  out  by  engineers  writing  from  a  more  or  less 
technical  point  of  view,  were  never  intended  as  matured  geological  opinions, 
and  it  would  be  unfair  to  treat  them  as  such. 


CHAPTER   III. 

LITHOLOGY. 

Section   1. 

THE  ROCKS  OP  THE  WASHOE  DISTRICT. 

Importance  of  lithology  to  the  theory  of  ore-deposits. ThoUgh     the     preSeilt     memoir    is 

intended  as  a  contribution  to  mining  geology,  the  importance  of  the  Hthology 
of  the'  district  is  certainly  not  less  than  it  would  be,  were  no  economical 
problems  involved.  The  slightness  of  the  advances  which  have  been  made 
in  the  theory  of  ore-deposits  is  regarded  by  business  men  as  a  reproach 
to  geological  science.  But  the  influence  of  the  inclosing  rocks  on  the  char- 
acter and  tenor,  and  to  some  extent  upon  the  occurrence  of  ore  bodies,  was 
recognized  before  geology  became  a  science;  and  the  fact  of  this  influence 
has  received  confirmation  from  more  extended  observation.  Whatever,  then, 
may  be  the  true  theory  of  the  genesis  of  ores,  the  indications  are  clear  that 
exhaustive  studies  of  the  nature  of  the  inclosing  rocks,  and  of  the  influences 
to  which  these  have  been  subjected,  are  essential  to  its  elucidation;  for  even 
if  it  should  prove  that  ores  are  derived  from  immense  depths,  and  are  brought 
to  the  surface  under  conditions  which  are  wholly  removed  from  observation 
and  study,  the  influence  of  the  wall  rocks  on  their  deposition  is  still  within 
the  accessible  field  of,  inquiry.  The  way  to  such  investigations  is  already 
paved.  The  microscopic  analysis  of  rocks  initiated  by  Mr.  Sorby,  and  raised 
to  its  present  rank  as  a  science  by  Messrs.  Vogelsang,  Zirkel,  Rosenbusch, 
Fouqud  &  L^vy,  and  their  fellow  workers,  enables  us  to  reach  very  definite 
conclusions  respecting  the  mineralogical  composition  and  ph3'sical  structure 
of  rocks;  while  Prof  F.  Sandberger  and  others  have  of  late  years  made  great 
advances  in  proving  the  chemical  relations  which,  in  many  cases,  exist 
between  the  wall  rock  and  the  contents  of  veins.  On  the  other  hand,  the 
mineralogical  study  of  decomposed  rocks  under  the  microscope  has  made 

32 


THE  EOCKS  OF  THE  WASHOE  DISTEICT.  33 

but  little  advance.  Geologists  who  do  not  deal  with  the  phenomena  of  ore 
deposits  are  commonly  satisfied  with  determining  the  species  of  the  rocks 
with  which  they  have  to  do,  and  recording  the  mere  fact  of  decomposition. 
They  therefoi-e  select  only  the  freshest  specimens  for  microscopical  exami- 
nation. If  the  I'esources  of  the  microscope  are  to  be  fully  brought  to  bear 
upon  the  study  of  ore  deposits,  mining  geologists  nmst  pursue  a  different 
method.  They  must  trace  the  mineralogical  course  of  decomposition-pro- 
cesses, and  learn  to  recognize  highly  altered  rocks,  even  when  fresh  speci- 
mens are  unattainable. 

Disputed  character  of  Washoe  rocks. — There  is  a  furtlicr  reasou  for  the  consider- 
able and,  as  it  may  seem  to  some  readei's,  the  undue  space  which  this  chap- 
ter occupies.  Baron  von  Richthofen  based  the  independence  of  the  new 
rock  propylite  largely  upon  the  occurrences  in  the  Washoe  District.  Later 
investigators  in  the  same  field  without  exception  have  adopted  his  views. 
Professor  Zirkel's  characterizations  of  the  microscopical  peculiarities  of  pro- 
pylite were  also  founded  chiefly  on  the  "Washoe  occurrence.  Though  at 
the  beginning  of  the  present  investigation  I  was  fully  persuaded  of  the  inde- 
pendence of  propylite,  I  subsequently  found  reason  to  doubt  it;  but  to  prove 
a  negative  is  notoriously  difficult,  and  the  great  authority  of  my  predeces- 
sors made  the  task  still  more  onerous.  It  was  necessary  to  demonstrate  that 
the  whole  superficial  area  and  all  the  accessible  mine-workings  were  occu- 
pied by  other  rock-species,  and  to  give  in  this  report  a  sufficient  number  of 
instances,  with  detailed  descriptions,  to  enable  geologists  to  decide  for  them- 
selves whether  the  elimination  of  propylite  and  the  redetermination  of  some 
other  rocks  is  justified  by  the  facts.^ 

1  Special  localities — The  rocks  of  the  Washoe  District  may  be  advantageously  studied  in  the  fol- 
lowing localities :  Gramilar  dioritc  in  nearly  all  varieties  occurs  along  the  line  of  the  Virginia  AVatcr 
Comjiany's  flume  within  a  distance  of  a  thousand  feet  north  of  Bullion  Eavine.  Poiylnjritio  diorites  can 
he  satisfactorily  examined  either  in  tbe  ilcKibbin  Tunnel  or  in  Oi)hir  Ravine,  hetween  the  most  west- 
erly point  of  the  flume  and  the  more  southerly  of  the  bluff's  marked  "  croppings"  on  the  map.  Earlier 
diabase,  in  all  varieties,  is  to  be  found  from  the  Savage  connection  with  the  Siitro  Tunnel  to  the  junc- 
tion of  the  maiu  tunnel  with  the  A'orlh  Lateral,  and  from  this  point  to  the  Mint  connection.  Younger 
diabafie  ("'  black  dike")  is  well  seen  on  the  west  wall  of  the  Belcher  associated  with  black  graphitic  slates. 
The  foregoing  are  the  rocks  most  important  to  miners  on  the  Lodh. 

•    Granite  is  well  developed  close  to  the  Eed  Jacket,  C.  D.  6,  and  on  the  dump  of  that  mine.     Quartz- 

porphyr>j  is  excellently  exi)osed  by  a  little  quarry  aboiit  2,000  feet  southwest  of  the  Justice.     The  felsitic 

variety  occurs  near  the  drainage  of  Gold  Canon  (American  Flat  Canon  is  the  name  given  on  former 

maps),  just  east  of  Roux'  Ranch.     The  little  basalt  mesa  in  the  same  locality  is  very  accessible.     Meta- 

3   C  L 


34  GEOLOGY  OF  THE  COMSTOCK  LODE. 


GEAIJITE. 

Character. — GrauitG  doBs  Hot  plaj  a  lai-g-e  part  in  the  geology  of  Washoe. 
Besides  the  small  area  laid  down  on  the  map,  it  has  been  struck  by  a  tunnel 
near  McClellan  Peak,  and  in  the  Rock  Island  and  Baltimore  mines;  so  far  a^, 
I  know,  nowhere  else  in  the  neighborhood.  The  rock  is  a  fine  typical  granite, 
consisting  of  orthoclase,  quartz,  biotite,  a  little  oligoclase,  magnetite,  and 
some  accessory  minerals.  The  apatites  are  colorless,  the  zircons  are  numerous 
and  beautiful,  and  the  titanite  occurs  in  typical  rhombs,  with  well  developed 
cleavages.  Finally,  it  contains  a  colorless  regular  mineral,  seemingly  in 
ill  developed  rhombohedrons,  which  answers  to  sodalite.  The  microscopi- 
cal characters  of  sodalite,  however,  are  rather  negative  than  positive,  and 
it  may  be  some  other  physically  similar  mineral. 

Near  the  Bed  Jacket  the  granite  shows  yQvy  distinct  parallel  partings, 
suggesting,  but  by  no  means  conclusive  of,  a  metamorphic  origin.  Some 
of  the  granite  has  been  mistaken  for  diorite,  and  a  part  of  the  metamorphic 
diorite  has  been  called  .granite;  but  these  are  errors  which  can  readily  be 
avoided  by  careful  inspection. 

ERUPTIVE  DIORITE. 

General  relations. — Tlic  devclopmcut  of  dioHte  in  thc  Washoe  District  is 
very  extensive,  and  the  variations  of  lithological  character  which  it  presents 

morpklc  diorite  occurs  close  to  tlie  granite.  It  is  found  as  a  very  volcanic  looking  breccia,  just  east  of 
the  Volcano  at  point  5,444,  C.  D.  6.  The  western  portion  of  the  small  patch  of  this  rock  in  C.  7  is 
extremely  similar  to  the  erupiive  diorite  of  Mount  Davidson.  Earlier  korntlcnde-andesite  in  a  fresh  state 
is  found  on  the  north  Twin  Peak  C.  T>.  4.  An  abandoned  quariy  500  feet  north  of  this  point  shows  the 
stages  of  its  decomposition  to  admiration.  The  south  Twin  Peak  is  an  occunence  of  loose  texture  and 
gray  color,  somewhat  resembling  the  younger  hornblende-audesite  of  the  Utah  neighborhood.  The  variety 
with  lai-ge  hornblendes  is  well  developed  at  point  5,678,  about  1,000  feet  east  of  the  Succor,  J).  5.  Other 
varieties,  including  decomposition-products  charged  with  epidote,  may  be  found  on  the  north  flank  of 
Cedar  Hill  Canon,  say  500  feet  west  of  the  Brewery.  Fresh  augiteandesite  can  be  conveniently 
examined  at  point  6,158,  close  to  the  Forman  shaft.  The  cuts  of  the  Occidental  Grade,  say  from  the  For- 
man  shaft  road  to  the  Prospect,  show  many  beautiful  examples  of  the  decomposition  and  disintegration 
of  blocks.  The  croppiugs  of  breccia  marked  6,569  on  the  Ophir  Grade,  B.  4,  show  many  transitions  and 
the  development  of  epidote.  Younger  hornbhndc-andesUe  is  found  as  a  purple  porphyry  at  the  quarries 
2,000  feet  northeast  of  Shaft  III.  of  the  Sutro  Tunnel;  as  a  red  porphyry  (very  augitic)  at  a  quarry  2,000 
feet  east  of  the  Occidental  mill ;  as  a  gray,  somewhat  granular  looking  mass  with  fine  columnar  structure 
in  the  quarry  close  to  the  Utah;  as  a  dense,  black,  glassy  rock  at  point  6,728  E  2.  The  tufa  modifica- 
tion is  best  seen  on  the  Sutro  road,  where  it  crosses  the  divide  between  Mount  Emma  and  Mount  Rose. 


THE  EOOKS  OF  THE  WASHOE  i)J STRICT.  35 

are  numerous,  and  often  perplexing.  While  the  varieties  often  differ  in 
appearance  from  one  another  much  more  than  is  the  case  with  separate 
species  of  the  younger  rocks,  there  is  strong  evidence  that  they  all  formed 
portions  of  a  single  extended  series  of  eruptions.  *  They  are  so  intermingled 
that  it  is  not  even  possible  to  lay  down  upon  the  map  distinct  areas  of  those 
which  differ  most,  but  it  seems  best  to  describe  the  principal  modifications 
separately,  and  afterwards  to  discuss  their  transitions. 

The  mass  of  Mount  Davidson  is  mainly  composed  of  granitoid  diorite 
of  a  cold  gray  color,  which  resembles  a  syenite  in  habitus  and,  as  has  been 
seen,  was  so  considered  until  Professor  Zirkel  demonstrated  the  triclinic 
nature  of  the  feldspars.  Two  other  modifications  of  the  granitoid  diorite 
require  attention.  One  of  them  is  a  very  dark  and  fine-grained  rock,  rep- 
resented to  a  alight  extent  upon  the  surface,  and  extensively  underground. 
It  has  sometimes  been  confounded  with  the  andesites.  The  other  is  a  coarse 
black  rock,  much  resembling  highly  graphitic  pig-iron.  It  has  been  found 
mostly  at  great  depths,  particularly  at  the  bottom  of  the  Union  shaft. 

Granitoid  diorite. — The  miueralogical  constituents  of  the  oi'dinary  light-gray 
and  the  dark,  fine-grained,  granular  diorites  are  essentially  plagioclase  and 
hornblende;  magnetite,  apatite,  and  zircon  seem  never  absent,  and  quartz, 
mica,  titanite,  and  augite  occur  now  and  then.  In  one  slide  tourmaline  has 
been  detected.  The  principal  constituents  seem  all  to  be  crystals  of  "sec- 
ondary consolidation;"  that  is  to  say,  they  have  all  formed  simultaneously 
on  the  cooling  of  the  rock,  and  have  mutually  interfered  with  one  another's 
growth,  EO  that  there  are  scarcely  any  symmetrically  developed  crystals 
present,  but  only  irregular  grains,  each  limited  by  surrounding  imperfect 
crystals  of  a  similar  character. 

The  hornblendes  are  generally  green  and  fibrous.  In  many  cases  the 
separate  fibers  appear  to  be  independent  microlites,  loosely  aggregated  in 
forms  characteristic  of  hornblende  crystals.  In  other  cases  they  appear  to 
be  distributed  entirely  without  reference  to  one  another.  The  impression 
produced  is  as  if  the  crystallization  had  taken  place  in  a  viscous  or  pasty 
mass,  which  mechanically  prevented  the  union  of  the  hornblende  molecules 
to  well  defined  crystals.  The  hornblendes  give  angles  of  extinction  appro- 
priate to  that  mineral,  when  the  well  known  variations  in  this  property  are 


36  GEOLOGY  OF  THP:  COMSTOCK  LODE. 

taken  into  consideration.  In  certain  localities  underground,  the  granular 
diorites  contain  much  deep-brown  and  solid  hornblende,  and  the  speci- 
mens which  show  this  variety  are  manifestly  fresher  than  those  from 
the  localities  wdiere  green  hornblende  occurs  exclusively.  In  some  cases 
an  alteration  of  the  brown  to  the  green  variety  is  strongly  suggested,  while 
in  one  series  of  porphyritic  diorites  it  can  be  actually  proved.  It  is  therefore 
altogether  probable  that  the  surface  diorite  originally  contained  some  brown 
hornblende,  which  has  been  changed  to  the  green,  fibrous  modification  by  a 
process  analagous  to  the  formation  of  uralite.  To  what  extent  the  fibrous 
hornblende  has  been  derived  from  the  brown  mineral,  there  is  at  present  no 
means  of  inferring. 

Augite. — Augite  is  comparatively  rare  in  the  unquestionable  granular 
diorites,  though  I  have  observed  it  in  a  few  instances.  It  is  much  more 
common  in  the  porphyritic  diorites,  and  it  may  be  that  its  absence  from  the 
granitoid  variety  is  due  to  conversion  into  uralite;  for  since  determinable 
crystal  sections  are  seldom  met  with  in  this  rock,  it  would  be  impossible  to 
distinguish  secondary  from  primary  green  fibrous  hornblende.  Close  to  the 
McKihhen  Tunnel  angular  fragments  of  what  appeared  macroscopically  to  be 
the  dark  fine-grained  diorite  frequently  encountered  in  the  district,  and 
especially  well  developed  in  this  tunnel,  were  found  embedded  in  light- 
colored  granular  diorite.  Under  the  microscope  the  inclosing  mass  exhibits 
no  peculiarity;  but  the  inclosed  rock,  unlike  the  similar  occurrences  in  the 
same  locality,  sliows  abundant  augite  and  almost  no  hornblende,  though 
structurally  resembling  the  dark  diorite.  As  the  distinct  diabase  eruptions 
are  manifestly  later  than  those  of  diorite,  I  am  wholly  at  a  loss  for  an  expla- 
nation of  this  case,  except  on  the  supposition  that  it  represents  a  local  and 
exceptional  substitution  of  augite  for  hornblende.  This  hypothesis,  how- 
ever, is  so  contrary  to  ordinary  experience  as  to  be  exceedingly  objection- 
able, though  were  it  true  it  would  also  serve  to  explain  the  ill- defined  patch 
of  diabasitic  rock  in  Ophir  ravine,  which  is,  like  that  just  mentioned,  much 
more  granitoid  than  the  mine  diabases,  and  has  no  apparent  structural  con- 
nection with  them.  There  can  be  httle  doubt  that  local  modifications  of 
massive  rocks  in  which  the  mineralogical  composition  is  characteristic  of  a 
distinct  but  allied  rock-species,  have  been  met  with  in  various  localities  in 


THE  BOOKS  OF  THE  WASHOE  DISTEIGT.  37 

the  world.  Such  cases,  however,  demand  very  cautious  treatment  at  the 
hands  of  the  geologist.  Actual  contacts  are  often  exceedingly  obscure,  and 
except  where  all  the  steps  of  a  transition  can  be  traced,  such  an  explanation 
of  an  anomalous  occurrence  is  not  justifiable.  Fortunately  nothing  further 
appears  to  depend  either  upon  the  specimen  of  diabasitic  fragments  inclosed 
in  a  dioritic  mass,  or  upon  the  diabasitic  area  in  Ophir  ravine,  since  the  evi- 
dence as  to  the  succession  of  the  rocks  in  the  east  country  is  decisive  and 
abundant. 

Other  constituents. — Tho  fcldspars  arc  nearly  or  quite  without  exception  tri- 
clinic,  and  simple  crystals  are  very  rare,  while  pericline  twinning  is  com- 
mon. The  stripes  indicating  polysynthetic  structure  are  usually  very 
well  defined,  and  of  moderate  width.  The  angles  of  extinction  of  a  very 
large  number  of  favorably  placed  crystals  have  been  noted,  and  seem  to 
indicate  labradorite  as  the  only  feldspar  present.  Zonal  structure  is  not 
uncommon,  but  the  feldspars  are  remarkably  free  from  inclusions  of  any 
kind,  and  are  in  general  thoroughly  transparent. 

Quartz  is  present  in  a  large  proportion  of  these  rocks,  though  its  dis- 
tribution is  very  irregular,  some  slides  containing  only  one  or  two  grains, 
while  others  show  hundreds  of  them.  Secondary  quartz  also  occurs,  but  it 
can  usually  be  distinguished  from  the  primitive  grains  with  ease.  Primitive 
quartz-grains  are  generally  single,  more  or  less  imperfectly  developed  crys- 
tals, around  which  grains  of  magnetite  and  other  small  crystals  are  so 
ari'anged  as  to  show  that  their  disposition  has  been  controlled  by  the  pres- 
ence of  the  quartz.  Secondary  quai'tz  occurs  in  veins  or  in  patches  composed 
of  granules  of  different  crj'stallographic  orientation,  and  is  not  sharply 
separated  from  the  surrounding  rock-mass.  Secondary  quartz,  of  course, 
frequently  carries  fluid  inclusions  in  rocks  of  all  ages.  The  primitive 
quartz  of  these  diorites  is  rich  in  liquid  inclusions,  some  of  them  vesicular 
in  shape,  and  others  dihexahedral.  The  smaller  ones  show  active  bubbles, 
and  in  some  slides  many  contain  salt-cubes.  I  have  noticed  none  of  the 
appearances  which  accompany  inclusions  of  carbonic  acid,  and  in  several 
slides  to  which  heat  was  applied  no  alteration  in  the  size  of  the  bubbles  was 
noticeable  at  a  temperature  considerably  above  40°  C. 

The  iron  ore  is  certainly  for  the  most  pai't  magnetite,  and  I  was  unable 


38  GEOLOGY  OF  THE  COMSTOCK  LODE. 

to  make  certain  of  any  ilmenite,  while  sphene,  in  small  and  irregular  masses, 
is  frequent.  Apatite  is  not  specially  plentiful,  and  is  of  the  ordinary  color- 
less variety.  Many  small  but  beautiful  zircons  are  visible  with  the  higher 
objectives,  mostly  in  eight-sided  prisms  terminated  by  the  fundamental 
pyramid. 

The  granitoid  diorites  resist  decomposition  better  than  any  other  rocks 
in  the  district.  On  the  surface  erosion  evidently  proceeds  with  greater 
rapidity  than  decomposition.  Slides  from  beneath  the  surface,  but  near  the 
Lode,  show  that  the  hornblende  is  replaced  by  chlorite  and  epidote,  and  the 
feldspars  by  calcite  and  quartz. 

Dark  varieties. — The  dark  fine-graiued  diorite  presents  a  much  stronger  con- 
trast to  the  ordinar}^  gi'^y  variety  macoscropically  than  microscopically.  The 
difference  in  its  appearance  seems  to  depend  simply' on  the  fineness  of  the 
grain,  and  on  the  percentage  of  fibrous  hornblende,  which  is  greater  in  this 
modification. 

The  dark  coarse-grained  dioi'ite  from  the  lower  levels  is  very  peculiar 
in  appearance,  and  some  of  that  from  the  Union  shaft  might  be  more  readily 
confounded  with  specimens  of  Scotch  foundry-pig  than  with  any  other  rock 
occurring  in  the  District.  This  variety  also  differs  from  the  ordinary  gray 
diorite,  principally  in  respect  to  the  hornblende,  which  is  more  abundant. 
It  is  not  fibrous  as  a  rule,  and  has  consolidated  in  grains  simultaneously  with 
the  feldspar.  As  in  the  freshest  gray  diorites,  the  granules  which  show 
no  evidences  of  alteration  are  brown,  and  incipient  alteration  seems  to  be 
accompanied  by  a  change  to  a  gi-een  fibrous  mass.  The  hornblendes  also 
contain  numerous  black  inclusions,  probably  ilmenite.  The  feldspars  are 
very  fresh  and  clear,  and  the  black  color  of  the  rock  is  the  natural  conse- 
quence of  such  a  mineral  composition.  Although  the  difference  in  appear- 
ance between  the  three  varieties  of  diorite  is  very  marked,  it  thus  depends 
on  a  variation  of  vinessential  characteristics. 

Structure  of  the  granular  diorites. — Tlic  granular  dioHtc  Is  exccedingl}^  hard  and 
tough,  so  much  so  that  before  the  introduction  of  nitro-g-lycerine  explosives 
it  was  almost  impossible  to  penetrate  it  where  decomposition  had  not  loosened 
the  texture.  In  the  Chollar  mine,  many  years  since,  when  black  powder 
only  was  in  use,  an  attempt  to  drive  a  gallery  into  this  rock  was  abandoned 


THE  EOCKS  OP  THE  WASHOE  DISTRICT.  39 

as  wholly  impracticable,  charge  after  charge  being  shot  from  the  drill-holes 
as  if  they  had  been  guns.  Under  tlie  hammer  it  exhibits  no  tendency  to 
break  in  one  direction  rather  than  in  another,  but  weathering  develops  con- 
siderable differences  in  resisting  power ;  and  in  Bullion  ravine,  as  may  be 
seen  from  Plate  VI.,  ridges  and  pinnacles  have  been  formed  by  the  irregu- 
lar disintegration  of  the  rock.  Immediately  west  of  the  Lode  the  diorite  is 
furthermore  divided  into  a  system  of  approximately  parallel  sheets.  It  will 
be  seen  in  the  next  chapter  that  I  refer  this  system  of  fissuring  to  a  faulting 
movement. 

Differences  from  other  rocks. — The  gray  grauular  dloHte  is  unlikely  to  be  con- 
founded with  any  other  rock  in  the  district,  except  granular  diabase.  This 
variety  of  diabase  seldom  occurs  underground,  so  far  as  the  country  is  now 
open  to  inspection ;  and  when  it  is  met  with,  as  at  the  Mint  connection  in 
the  Sutro  Tunnel,  it  is  commonly  limited  to  a  very  small  body  which  shades 
off  into  finer-grained  varieties.  When  decomposition  has  progressed  too 
far  to  permit  a  macroscopical  determination  of  the  mineral  constituents,  the 
lath-like  development  of  the  feldspars,  the  tendency  to  cleavage  in  parallel 
planes,  and  a  certain  waxy  luster  will  usually  be  found  characteristic  of  the 
diabase.  The  dark  fine-grained  diorite  has  repeatedly  been  taken  for  ande- 
site  in  the  McKihhen  Tunnel  and  elsewhere.  The  only  resemblance,  how- 
ever, is  in  color,  for  the  diorite  shows  to  the  naked  eye  a  grannlai-  structure 
never  observed  in  the  andesites  of  the  District,  although  the  latter  ai-e  un- 
commonly crystalline. 

Porphyritic  diorites. — From  some  peculiainty  either  in  composition  or  texture, 
the  porphyritic  hornblende-diorites  have  undergone  very  extensive  decom- 
position, and  it  was  only  after  long  and  earnest  search  that  two  or  tin-ee 
small  masses  were  found,  which  might  furnisli  a  study  of  this  diorite  in 
a  fresh  state.  A  close  inspection  of  fresh  specimens  shows  that  the  rock 
is  even  macroscopically  thoroughly  crystalline,  but  that  tolerably  well-devel- 
oped feldspars  and  good  hornblendes  are  separated  out  in  a  finer  ground- 
mass  of  a  dark  color.  In  addition  to  these  minerals  the  microscope  sliows 
magnetite,  apatite,  and  zircon.     Augite  and  mica  also  occur  in  limited  areas. 

The  hornblendes  when  fresh  are  bright  brown  and  well  crystallized, 
often  showing  terminal  faces  as  well  as  the  prism  and  clinopinacoid.     In  a 


40  GEOLOGY  OF  THE  COMSTOCK  LODE. 

slide  from  Cedar  Hill  the  curious  inclusions  which  seem  probably  ilmenite 
needles,  already  referred  to,  are  developed  in  great  perfection.  Most 
of  the  hornblende  substance  is  concentrated  in  the  larger  crystals,  but 
there  are  a  few  minute  ones  and  some  crystalline  fragments  interspersed 
through  the  groundmass.  The  larger  feldspars  are  fairly  well  developed, 
but  have  not  the  sharpl}^  rectilinear  outlines  so  common  in  the  diabases  and 
the  volcanic  rocks,  nor  do  they  display  any  tendency  to  elongated  lath-like 
forms.  They  give  the  angles  of  extinction  appropriate  to  labradorite;  they 
contain  occasional  fluid  inclusions,  of  rounded  forms,  and  of  course  no  glass. 
They  are  pierced  by  numerous  apatite  needles.  The  smaller  feldspars  are 
in  part  crippled  grains,  similar  to  those  of  the  granitoid  diorites,  and  in 
part  elongated  microlites.  The  angles  of  extinction  of  these  latter  render 
it  probable  that  they  are  oligoclase. 

The  iron  ore  seems  to  be  exclusively  magnetic.^  The  apatite  is  in  part 
of  the  ordinary  colorless  variety,  and  in  part  brown  and  dusty.  The  inde- 
terminable inclusions  in  the  apatites  are  disposed  very  differently  in  different 
individuals.  An  hexagonal  brown  core  is  sometimes  surrounded  by  color- 
less apatite,  while  in  other  cases  this  arrangement  is  reversed.  Longitudinal 
sections  not  infrequently  show  colorless  ends,  with  a  dusty  middle  portion. 
Only  a  few  zircons  have  been  observed  in  this  rock,  in  which  respect  it  dif- 
fers from  the  granular  diorites.  The  groundmass  consists  of  small  feldspars 
and  magnetite  grains,  and  its  general  effect  is  usually  that  of  an  excessively 
fine-grained  granitoid  diorite.  Occasionally  the  arrangement  of  the  micro- 
lites is  such  as  to  suggest  fluidal  structure. 

Decomposition. — The  decompositlou  of  these  rocks  forms  an  exceedingly 
interesting  study.  It  will  be  shown  elsewhere  in  detail  that  the  hornblendes 
pass  into  chlorite,  and  this  again  into  epidote,  quartz,  and  calcite.  The  chlo- 
rite evidently  possesses  a  high  degree  of  solubility,  and  soon  diffuses  itself 
through  the  groundmass,  and  through  the  feldspars  so  far  as  these  latter  have 
become  porous  from  decomposition.  The  chlorite  and  epidote  give  the  par- 
tially decomposed  rocks  their  characteristic  greenish  hue. 

Another  change  of  great  interest  appears  in  a  small  dike  of  porphyritic 
diorite  cutting  granular  diorite  close  to  the  Eldorado  croppings.  No  effect 
whatever  has  been  produced  upon  the  inclosing  granular  diorite,  but  for 

'Excepting  the  acicular  inclusions  referred  to  above. 


THE  ROCKS  OF  THE  WASHOE  DISTRICT.  41 

about  an  inch  from  the  edge  the  intrusive  porphyritic  rock  has  an  exces- 
sively fine  grain  and  close  texture.  In  consequence  of  this  physical  char- 
acter it  has  resisted  decomposition,  and  close  to  the  contact  is  very  fresh. 
Here  it  contains  fine  brown  hornblende,  but  at  a  distance  of  half  an  inch 
from  the  contact,  the  texture,  as  seen  under  the  microscope,  becomes  coarser 
and  more  open,  and  green  fibrous  hornblende  makes  its  appearance.  Cer- 
tain hornblende  individuals  are  brown  towards  the  center,  but  gi'een  and 
fibrous  near  the  edges  and  along  cracks,  and  the  dividing  line  is  such  as  to 
leave  no  doubt  that  in  this  case  the  green  fibrous  modification  is  to  be 
regarded  as  an  alteration-product  of  the  brown  dense  variety.  This  occur- 
rence strongly  confirms  the  indications  of  such  a  transformation  mentioned 
in  describing  the  granular  diorites.^ 

Structure  of  porphyritic  diorites. — No  special  tcudeucy  to  parting  in  any  direction 
is  perceptible  in  the  porphyritic  diorites,  but  they  vary  in  coarseness  of 
grain  and  general  appearance  much  more  than  the  granitoid  diorites.  To 
the  south  of  Bullion  ravine  there  are  small  localities  where  the  rock  is  dis- 
tinctly brecciated ;  and  in  the  ravine  west  of  the  Imperial  there  is  a  small 
occurrence  of  excessively  fine-grained  diorite,  with  a  closely  laminated 
structure,  not  unlike  a  calcareous  slate.  Similar  spots  are  found  in  the 
diabase  and  the  andesites,  but  in  no  case  was  any  explanation  apparent  from 
the  character  oT  the  surroimding  masses.  Some  such  appearance  might 
ensue  in  a  pasty  mass  if  its  compo.sition  were  locally  altered  and  its  fusibility 
increased,  say  by  the  presence  of  a  fragment  of  calcareous  rock.  There  is 
no  relation  between  the  direction  of  the  lamellae  of  these  spots  and  the 
general  fissure  system.  Where  decomposition  has  proceeded  far  enough 
for  a  diffusion  of  chlorite  through  the  rock  to  take  place,  the  granular  texture 
of  the  groundmass  is  much  obscured,  and  the  similarity  between  it  and 
certain  partially  decomposed  andesites  is  great  and  misleading.  A  large 
portion  of  what  was  supposed  to  be  propylite  in  the  Washoe  District,  is 
porphyritic  diorite  in  this  stage  of  decomposition.  Disintegration  sometimes 
accompanies  decomposition  of  the  rock,  but  an  astonishing  coherence  is  often 
maintained  when  scarcely  a  particle  of  unaltered  mineral  is  left. 

Diagnostic  points. — It  is  ofteu  vcry  difficult  to  distinguish  between  partially 

'  Confer  Eosenbusch,  Physiog.  iler  Min.  u.  Gest.,  Vol.  II.,  p.  :i33. 


42  GEOLOGY  OF  THE  COMSTOCK  LODE. 

decomposed  diorite  and  hornblende-andesite ;  and  the  only  really  safe  course 
is  to  continue  the  examination  until  comparatively  fresh  specimens  are 
obtained.  The  granular  structure  of  these  is  not  readily  confounded  with 
that  of  andesite.  The  diorites  are  never  fissile  like  hornblende-andesite,  and 
hornblende-andesite  is  usually  pretty  uniform  over  considerable  areas,  while 
the  dioritic  porphyries  var)^  in  structure  almost  from  yard  to  yard. 

Mica-diorites. — Tlic  micaccous  diorite-porphyries  do  not  differ  greatly  from 
the  hornblendic  variety,  except  in  the  substitution  of  biotite  for  hornblende; 
but  the  rock  is  of  a  looser  texture,  the  porphyritic  feldspars  are  generally 
larger,  and  tend  more  to  rounded  forms.  I  met  with  no  occurrence  of  this 
variety  in  a  fresh  condition. 

Relations  of  the  diorites. — All  varietics  of  dloHte  pass  over  into  one  another. 
Porphyritic  and  micaceous  forms  occur  in  the  prevailing  granular  mass  on  the 
front  of  Mount  Davidson,  directly  opposite  the  Savage  mine,  and  granitoid  • 
diorites  occur,  mixed  with  the  porphyritic  forms,  on  Cedar  Hill  and  in  the 
McKihhen  Tunnel.  Especially  in  the  latter  locality  gradations  of  the  one  form 
into  the  other  can  be  excellently  followed  out.  The  evidence  of  this  character 
is  sufficient  to  prove  conclusively  that  no  absolute  separation  can  be  estab- 
lished between  the  dioritic  rocks.  There  is,  however,  also  considerable  evi- 
dence to  show  that,  as  a  whole,  one  variety  succeeded  another  in  the  course 
of  the  eruption  of  the  mass.  The  first  portion  of  the  diorites  appears  to 
have  been  of  the  dark,  fine-grained  variety,  and  cases  have  been  met  with 
in  which  dikes  of  the  lighter  rock  cut  the  darker.  In  the  mines,  too,  the 
excavations  show  that  the  dark  rock  frequently  underlies  the  lighter,  and 
in  the  deepest  workings  the  dark  predominates  over  the  light  rock.  There 
ai-e  not  sufficient  exposures  of  the  coarse-grained  diorites  with  brown 
hornblendes  to  determine  its  relations  to  the  varieties  with  fibrous  horn- 
blendes, but  it  evidently  preceded  the  poq^hyritic  rocks.  The  main  mass 
of  the  porphyritic  diorite  succeeded  the  granitic.  Just  to  the  south  of 
the  Eldorado  croppings  there  is  a  distinct  dike  of  porphyritic  diorite  in 
the  granitic  mass,  with  well-developed  contact  phenomena,  extending  about 
an  inch  from  each  wall  of  the  dike.  To  the  north  of  Mount  Davidson  the 
mine  shafts,  too,  have  gone  down  through  porphyritic  diorite  into  the  granu- 
lar variety.     Indeed  Mount  Davidson,  between  Bullion  ravine  and  Spanish 


THE  EOCKS  OF  THE  WASHOE  DISTRICT.  43 

ravine,  constitutes  the  principal  area  of  the  granitic  diorite  at  tte  surface; 
while  both  to  the  north  and  the  south  porphyritic  varieties  prevail,  and 
nearly  all  the  diorite  to  the  east  of  the  Lode  is  of  the  same  character. 


METAMORPHIC    DIORITE. 

Origin  and  association. — Tlic  soutliem  portiou  of  the  dlstrict  contains  a  large 
area  of  this  rather  puzzling  rock.  It  was  mentioned  by  Mr.  King  as  a  "com- 
pact, black,  crystalline  rock,  which  in  hand  specimens  would  unquestionably 
be  classed  as  a  basalt,"  but  which  can  be  shown  to  be  of  metamorphic  origin. 
Professor  Zirkel  determined  it  as  a  peculiar  basalt.  The  most  ordinary 
variety  is  of  a  black  or  iron-gray  color,  and  shows  an  irregular  crystalline 
fracture;  but  certain  varieties  (the  more  feldspathic  ones)  are  light  in  color, 
and  considerably  resemble  Mount  Davidson  diorite.  There  is  no  little  diffi- 
culty in  determining  whether  this  rock  shall  be  regarded  as  of  metamorpliic 
origin,  or  as  eruptive.  To  the  west  of  the  Florida  mine  the  contact  between 
it  and  the  underlying  metamorphic  rocks  appears  as  sharp  as  possible.  At 
the  Wales  Consolidated,  a  mine  opened  on  a  deposit  lying  between  this  rock 
and  the  granite,  there  is  no  evidence  whatever  of  bedding.  Near  the  Amazon 
mine  it  is  weathered  in  round  boulders,  precisely  like  those  produced  by  the 
action  of  frost  on  basalt;  and  close  to  the  Volcano  mine  it  forms  a  distinct 
bi'eccia.  On  the  other  hand,  in  some  of  the  railroad  cuts,  there  appear  to 
be  transitions  into  rocks  of  evidently  sedimentary  origin.  But  such  appear- 
ances need  very  cautious  treatment,  for  between  metamorphism  and  decom- 
position, a  contact  might  readily  assume  the  appearance  of  a  transition.  The 
microscopical  character  of  the  rock  offers  nothing  decisive  as  to  its  origin, 
and  the  point  which  has  mainly  determined  me  to  regard  it  as  metamorphic 
is  its  relation  to  the  quartz-porphyry.  An  inspection  of  the  map  will  show 
that  it  is  invariably  associated  with  the  quartz-porphyry,  and  that  if  it  had 
resulted  from  the  metamorphism  of  the  sedimentary  strata  by  porphyry 
eruptions,  subsequent  erosion  must  have  exposed  it  in  relations  almost  iden- 
tical with  those  observed.  Its  composition  also  indicates  a  metamorphic 
rather  than  an  eruptive  origin. 


44  GEOLOGY  OF  THE  CO]\ISTOCK  LODE. 

Character  in  detail. — The  pniicipal  constitiieiits  are  plagioclase,  hornblende, 
and  mica,  often  with  the  addition  of  quartz;  wliile  the  subsidiary  minerals 
are  titanic  iron,  apatite,  sphene,  zircon,  and  in  one  case  tourmaline.  The 
hornblende  is  for  the  most  part  fibrous  and  bluish  green.  But  this  does  not 
appear  to  have  been  its  original  color.  The  centers  of  considerable  masses 
of  hornblende  appear  under  the  microscope  wholly  colorless,  and  the  asso- 
ciation of  the  two  varieties  described  in  detail  under  slide  295  is  such  as  to 
lead  almost  inevitably  to  the  conclusion  that  the  green  color  is  secondary. 
In  many  specimens  the  hornblende  is  pi-esent  in  great  quantities,  and  micro- 
lites  so  crowd  the  feldspars  that  their  striations  are  almost  imperceptible  and 
their  species  indeterminable.  To  a  considerable  extent  the  hornblende  is 
decomposed  into  chlorite  and  epidote,  the  latter  mineral  appearing  in  unu- 
sually fine  crystals.  Mica  is  present  in  smaller  quantities  than  hornblende, 
and  gives  the  interference  figvire  of  biotite.    Some  specimens  contain  augite. 

In  the  less  hornblendic  varieties  the  feldspars  are  well  developed,  many 
of  them  with  sharp,  rectilinear  outlines,  like  those  in  the  more  porphyritic 
diabases.  The  lamellae  are  exceedingly  attenuated;  they  show  angles  of  ex- 
tinction appropriate  to  oligoclase,  and  pericline  twinning  is  occasionally 
visible.  A  few  orthoclase  crj'stals  also  occur.  Quartz  grains  are  numerous 
in  the  less  hornblendic  specimens,  and  do  not  appear  to  be  of  secondary 
origin.  They  contain  numerous  nn'nute  fluid  inclusions,  and  with  irreg- 
ular grains  of  feldspar  form  a  kind  of  coarse  groundmass.  The  titanic  iron 
is  accompanied  by  leucoxene,  which  in  some  cases  appears  to  pass  over  into 
titanite,  though  a  transition  cannot  be  demonstrated.  The  apatite  presents 
nothing  peculiar,  and  the  zircon  is  in  no  way  remarkable  except  in  the  fre- 
quency of  its  occurrence.  Tourmaline  was  found  onl}-  in  one  slide.  Of 
course  it  suggests  metamorphic  origin,  though  the  same  mineral  is  known 
to  occur  occasionally  in  rocks  of  eruptive  origin,  and,  as  has  already  been 
mentioned,  was  noticed  in  one  of  the  almost  unquestionably  eruptive 
diorites  of  this  very  district. 

Comparatively  little  decomposition  has  been  noticed  in  this  rock,  a  fact 
which  no  doubt  stands  in  intimate  relation  to  its  unusual  hardness  and 
toughness,  but  in  some  Hmited  areas  it  is  highly  chloritic,  and  certain  speci- 
mens would  pass  for  propylite. 


THE  ROCKS  OP  THE  WASHOK  DISTRICT.  45 

Diagnostic  peculiarities. — SoiTie  Varieties  of"  tliis  vock,  especially  a  portion  of 
the  small  patch  shown  on  the  map  in  square  C.  7,  greatly  resemble  Mount 
Davidson  diorite,  and  indeed  the  difference  under  the  microscope  is  chiefly 
in  the  species  of  feldspar.  On  the  hills  west  of  the  Florida,  and  in  some 
other  localities,  it  is  much  like  augite-andesite  or  basalt  in  appearance,  but 
the  macroscopical  resemblance  does  not  answer  to  any  microscopical  simi- 
larity. It  sometimes  occurs  in  rounded  shapes  such  as  basalt  often  assumes. 
In  many  cases  weathered  surfaces  of  this  rock  can  be  recognized  by  the 
crystal  outlines  which  they  exhibit.  These  are  often  polygons  of  a  variable 
number  of  sides,  and  represent  sections  of  hornblendes  crystallized  in  re- 
markably short  prisms  and  provided  with  terminal  faces. 


QUARTZ-PORPHYRY. 

General  ciiaracter. — Quartz-jjorpliyry  covers  a  large  area  in  the  southwestern 
portion  of  the  Washoe  District,  and  extends  for  miles  in  the  direction  of 
Washoe  Lake.  It  presents  a  rough  surface  varying  in  color  from  white  to 
a  yellowish  or  reddish  gray,  and  is  thickly  set  with  quartz-grains  the  size 
of  a  mustard  seed,  and  smaller.  Mica  is  nearly  always  visible,  and  horn- 
blende occasionally.  Only  in  one  small  area  near  the  granite  is  the  quartz 
macroscopically  suppressed,  and  here  the  rock  is  finer-grained  than  else- 
where. Underground  it  extends  to  a  considerable  distance  farther  north  and 
east  than  its  northern  limit  on  the  surface,  and  there  underlies  hornblende- 
andesite  and  augite-andesite. 

Composition. — Nouc  of  thc  rock  is  really  fresh  and,  though  an  earnest 
search  was  made,  not  a  single  specimen  could  be  found  showing  the  con- 
stituent minerals  in  an  undecomposed  state.  Those  which  enter  most  largely 
into  the  composition  of  the  rock  are  feldspar,  quartz,  mica,  hornblende,  and 
ores  of  iron.  As  accessory  minerals,  titanite,  apatite,  and  zircon  were 
observed.  The  feldspars  are  not  well  defined,  but  occur  in  irregular  or 
rounded  grains.  They  are  in  part  striated,  but  the  larger  portion  show  no 
trace  of  polysy nthetic  structure,  and  while  in  some  slides  so  large  a  quantity 
of  calcite  is  distributed  through  the  feldspars  that  the  striations  might  be 


46  GEOLOGY  OP  THE  COMSTOCK  LODE. 

supposed  to  be  obliterated,  in  others  the  crystals  are  so  clear  that  striations, 
if  present,  could  not  but  be  apparent.  Many  of  the  unstriated  feldspars  show 
cleavages,  and  extinguish  light  at  angles  which  seem  to  prove  their  ortho- 
clastic  character.  No  unstriated  feldspars  were  found  to  give  angles  of 
extinction,  reckoned  from  the  cleavage  planes,  which  would  refer  them  to 
either  of  the  triclinic  species.  The  triclinic  crystals  show  for  the  most  part 
very  narrow  striations,  and  give  angles  of  extinction  which  correspond  to 
oligoclase.  The  microlitic  feldspars  of  the  groundmass  do  not  appear  to  be 
triclinic.  On  introducing  a  portion  of  the  rock  in  a  condition  of  fine  powder 
into  the  well-known  solution  of  mercuric  iodide  in  potassic  iodide,  of  a 
specific  gravity  of  less  than  2.65,  a  large  proportion  rose  to  the  surface. 
This  mounted  in  balsam  ajjpeared  to  consist  mainly  of  feldspar.  The  por- 
tion which  sank  contained  some  feldspar  and  the  other  components  of  the 
rock.  The  feldspars  contain  inclusions  of  glass  and  also  of  fluid,  but  a  por- 
tion of  the  latter  I  regard  as  of  secondary'  oi-igin. 

Quartz. — The  quartzes  are  bounded  in  part  by  straight  lines  and  in  part 
by  curves.  In  some  cases  the  imperfectly  developed  crystals  appear  to 
have  been  broken,  and  the  fragments  are  now  separated  by  narrow  bands 
of  groundmass;  in  other  cases  they  contain  deep  sinuous  bays  of  the  same 
material.  The  quartz  shows  both  fluid  and  glass  inclusions,  but  their  distri- 
bution is  somewhat  uneven.  In  some  slides  they  are  present  in  nearly  equal 
numbers,  while  in  others  one  or  the  other  preponderates,  or  even  occurs 
exclusively;  but  this  is  exceptional.  The  inclusions  are  not  thicklj^  set,  but 
a  glass  inclusion  and  a  fluid  inclusion,  with  a  moving  bubble,  can  often  be 
seen  in  the  same  field  with  a  Hartnack  No.  7  objective.  Of  the  hornblendes, 
which  were  all  black-bordered,  none  now  i-emain  in  a  fresh  condition. 
They  have  been  replaced  by  the  usual  products  of  decomposition — chlorite, 
epidote,  quartz,  and  calcite.  The  mica,  too,  is  in  great  part  decomposed, 
but  occasional  scales  remain,  and  these  give  the  interference  figure  of  bio- 
tite.  Tlie  groundmass  in  every  case  shows  fluidal  and  pseudospherolitic 
structure.  In  some  cases  a  base  is  also  present;  in  others  it  is  either  wanting 
or  devitrified.  When  glass  is  present,  it  shows  a  preference  for  elongated 
sinuous  forms,  and  often  the  central  line  is  marked  by  aggregations  of  iron 
ore  from  which,  as  axes,  black  trichites  sometimes  spread  into  the  sun'ounding 


THE  EOCK,S  OP  THE  WASHOE  UISTEIOT.  47 

isotropic  substance.  The  iron  ore  is  in  part  magnetite,  while  in  other  cases 
it  appears  to  be  ilmenite.  Apatites  of  the  usual  colorless  variety  are  fre- 
quent. Zircons  are  not  uncommon,  and  there  are  occasional  small  patches 
of  titanite. 

Field  habit. — The  croppings  of  the  quartz-porphyry  are  usually  exceed- 
ingly rough,  and  the  nearest  a2)j)roach  to  a  structure  is  indicated  in  some 
localities  by  the  separation  of  the  rock  into  uneven  sherdy  fragments.  Its 
appearance  is  almost  identical  all  over  the  district,  except  in  the  small  area 
where  the  quartz  is  macroscopically  suppressed.  Here  it  shows  various 
brown  and  green  colors,  and  sometimes  a  smooth  fracture  like  a  fine-grained 
hornblende-andesite.  In  this  area  the  color  and  texture  vary  every  few  feet. 
This  macroscojiical  difference  appears  to  correspond  to  no  microscoiDical 
peculiarity  beyond  a  finer  grain.  As  quartz-porphyry  is  the  only  quartzose 
rock  in  the  district,  it  is  readily  distinguishable. 

Various  determinations. — As  has  bccn  Seen  in  the  rdsumd  of  former  memoirs, 
the  quartz-porphyry  has  been  variously  determined  by  the  eminent  geolo- 
gists who  have  discussed  the  Washoe  District.  Baron  v.  Richthofen  very 
positively  asserted  that  the  circumstances  of  its  occurrence  rendered  it  cer- 
tain that  this  porphyry  was  intermediate  in  age  between  the  granitic  and  the 
volcanic  rocks,  and  I  entirely  agree  with  him.  The  absolute  uniformity  of 
the  rock  fi'om  the  Overman  mine  to  the  southern  extremity  of  the  mass,  with 
the  exception  of  a  small  felsitic  area,  utterly  precludes  the  supposition  that 
it  is  separable  into  different  species  of  Tertiary  and  pre-Tertiary  origin. 
The  felsitic  modification  comes  in  contact  only  with  granite  and  basalt,  but 
its  microscopical  character  is  identical  with  that  of  the  coarser  porphyry;  it 
strongly  resembles  well-known  varieties  of  quartz-porphyry,  and  I  can  see 
no  evidence  on  the  ground  sufficient  to  separate  it  from  that  species.  That 
in  the  Overman  and  Caledonia  mines  and  the  Forman  shaft  the  porphyry 
vertically  underlies  hornblende-andesite  is  beyond  question;  both  optical 
tests  and  specific  gravity  determinations  show  that  it  is  an  orthoclase  rock; 
and  the  character  and  association  of  the  inclusions  in  the  quartzes  are  pre- 
cisely those  which  are  so  very  common  in  old  quartz-porphyry.  Professor 
Zirkel  determined  the  larger  proportion  of  this  rock  as  a  dacite,  but  on 
reexamining  his  slides  I  found  that  they  corresponded  in  every  respect  to 


48  GEOLOGY  OP  THE  COMSTOCK  LODE. 

mine,  and  that  the  quartzes  in  each  of  them  contained  fluid  inclusions  with 
moving  bubbles.  The  one  slide  which  Professor  Zirkel  determined  as  rhy- 
olite  differs,  in  that  the  quartz  contains  glass  but  no  fluid  inclusions;  in  a  slide 
of  my  own,  however,  from  as  nearly  as  possible  the  same  locality,  these  con- 
ditions are  reversed,  the  quartzes  showing  fluid  inclusions  but  none  of  glass. 
I  can  see  no  difi"erence  in  the  amount  of  orthoclase  present  in  those  slides 
determined  respectively  as  dacite  and  rhyolite.  Professor  Zirkel  gives  an 
analysis  of  this  rock  made  by  Mr.  Councler,  showing  two  per  cent,  of  soda 
and  three  and  six-tenths  per  cent,  of  potash.  In  discussing  this  composition 
Professor  Zirkel  cites  a  number  of  analyses  of  Transylvania  dacites,  but  in 
none  of  these  is  the  proportion  of  potash  to  soda  so  high  as  in  the  Washoe 
rock. 

DIABASE. 

Earlier  diabase. — There  are  two  varieties  of  diabase  in  the  district.  The 
older  of  these  forms  the  hanging  wall  of  the  Lode;  the  other  has  beeri 
known  as  "black  dike."  The  east-country  diabase  varies  considerably  in 
coarseness  of  grain  and  in  color.  When  really  fresh  it  is  always  dark,  and 
when  also  fine-grained  it  closely  resembles  an  andesite.  The  coarser-grained 
and  somewhat  decomposed  occurrences  are  often  confusingly  like  granitoid 
diorite. 

The  rock  consists  of  plagioclase,  augite,  and  an  iron  ore,  with  a  num- 
ber of  accessory  and  irregularly  distributed  minerals,  quartz,  hornblende, 
mica,  and  apatite.  The  structure  is  not  that  most  usually  found,  in  diabases, 
being  somewhat  porphyritic.  The  augite  is  of  the  usual  pale-brown  tint, 
and  occurs  largely  in  well-developed  crystals.  These  are  often  twinned 
according  to  the  ordinary  la,w.^  The  twinning  attracts  more  attention  than 
usual,  because  polysynthetic  structure  is  common,  some  of  the  lamellse  often 
penetrating  only  part  way  through  the  crystal.  The  ordinary  cleavages  are 
well  marked,  and  instances  are  common  in  which  the  pinacoidal  cleavages 
as  well  as  the  prismatic  ones  are  developed.  Some  slides  contain  only  sepa- 
rate and  well-formed  crystals,  while  in  others  they  occur  in  groups,  and 
these  are  apt  to  be  gathered  about  branching  masses  of  iron  ore,  almost  like 

'  For  a  peculiar  case,  which  might  be  interpi*ted  as  abnormal,  see  page  113. 


THE  ROOKS  OF  THE  WASHOE  DISTRICT.  49 

close-growing  bunches  of  grapes.  In  still  other  slides  grains  of  the  mineral 
are  distributed  through  the  groundmass. 

Mineral  constituents  in  detail. — Tliis  rock  alwRjs  contains  porpliyritical  feld- 
spars. They  are  long,  sharply  rectilinear,  and  without  exception  triclinic. 
They  give  angles  of  extinction  pi-oper  to  labradorite.  The  lamellae  are  of 
moderate  width,  and  are  often  combined  at  the  same  time  according  to  all 
the  common  twinning  laws.  In  nearly  every  slide  they  carry  liquid  inclu- 
sions, generally  of  vesicular  shapes.  The  smaller  feldspars  form  granitoid 
grains  of  "secondary  consolidation,  "and  with  the  iron  ores  and  more  or  less 
augite,  make  up  the  groundmass.  I  have  observed  some  of  these  smaller 
feldspars  which  gave  angles  of  extinction  indicating  a  different  species  from 
the  larger  crystals  of  first  consolidation.  The  iron  ore  is  in  part  magnet- 
ite, and  in  part  ilmenite,  with  the  characteristic  cleavage-lines  and  products 
of  decomposition. 

Quartz  grains  of  unquestionably  primitive  character  are  occasionally 
met  with.  These  show  an  arrangement  of  particles  of  magnetite,  etc., 
about  their  peripheries  such  as  secondary  quartzes  never  exhibit.  Almost 
all  of  them  show  fluid  inclusions,  the  smaller  ones  with  moving  bub- 
bles. I  have  observed  none  in  which  the  liquid  appeared  to  be  in  the 
spheroidal  state,  and  the  bubbles  do  not  disappear  at  a  temperature  of 
above  40°  C;  the  fluid  is  therefore  aqueous.  I  have  met  with  no  salt 
cubes.  Hornblende  occurs  sparingly,  and  is  generally  confined  to  closely- 
limited  areas.  Where  it  is  present  great  care  is  necessary  in  discriminating  the 
rock  macroscopically  from  diorite.  Mica  is  rare,  and  is  seen  only  in  almost 
indeterminably  small  particles,  which  might  even  be  secondary.  The 
apatite  is  of  the  usual  colorless  variety.     Not  a  single  zircon  was  detected. 

Evidences  of  diabasitic  character. TllO     mlcrOStrUCtUrC   of    this  I'Ock   StrOUgly  SUg- 

gests  that  of  some  lavas,  and  I  have  sometimes  been  puzzled  to  say  at  the 

first  glance  whether  a  particular  slide  was  augite-andesite  or  diabase;  but 

the  resemblance  is  superficial.     As  will  be  seen  later,  somewhat  granular 

augite-andesites  occur  in  the  district,  but  they  are  exceptional.     Here  as 

elsewhere  the  younger  rock  generally  shows  a  microlitic  groundmass,  and 

frequently  a  glass  base.     This  is  the  case  equally  on  the  surface,  and  in  the 

Sutro  Tunnel  more  than  a  thousand  feet  beneath  the  surface.    The  diabase  now 
5  0  L 


50  GEOLOGY  OF  THE  COMSTOCK  LODE. 

under  discussion  shows  in  all  cases  a  thoroughly  crystalline  structure,  and 
the  groundmass  is  always  composed  of  granitoid  grains.  The  feldspars  of, 
I  believe,  every  slide  of  the  augite-andesite  show  glass  inclusions;  and  I 
have  not  met  one  fluid  inclusion  in  that  rock  which  appeai'ed  to  me  of  prij 
mary  origin.^  In  the  diabase  the  occurrence  of  fluid  inclusions  and  the 
absence  of  those  of  glass  is  equally  universal.  The  auglte  of  the  augite- 
andesites  shows  no  pinacoidal  cleavages,  and  only  one  locality  has  been 
detected  at  Washoe  in  which  it  has  passed  into  uralite.  The  change  even 
there  is  so  exceedingly  local  that  although  a  dozen  slides  have  been  ground 
from  the  same  cropping,  but  one  shows  the  alteration  of  augite  into  horn- 
blende. In  the  diabase  the  passage  of  augite  into  uralite  is  the  usual  pre- 
liminary to  chloritic  decomposition.  Finally,  if  there  is  one  point  of  struct- 
ure incapable  of  two  intei'pretations,  it  is  that  the  black  dike  is  of  later 
origin  than  the  east  and  west  country  rocks.  As  will  be  shown,  the  black 
dike  is  an  ordinary  diabase,  and  the  hanging  wall  is  consequently  a  pre- 
Tertiary  rock,  and  would  necessarily  be  classed  as  a  diabase  were  its  resem- 
blance to  the  volcanic  series  much  more  thorough  than  it  really  is. 

Decomposition. — lu  dccomposiug,  the  diabase  shows  few  peculiarities.  As 
has  already  been  mentioned,  the  augite  is  apt  to  be  converted  into  uralite 
and  then  into  chlorite.  Epidote  almost  always  forms  to  some  extent  from 
the  chlorite,  but  the  latter  does  not  generally  seem  to  pass  so  readily  and 
completely  into  epidote  as  does  that  which  results  from  the  degeneration  of 
hornblende.  Instances  occur,  however,  where  the  conversion  is  complete. 
The  decomposition  of  the  feldspars  presents  no  peculiarity.  They  change 
slowly  to  quartz  and  calcite,  and  become  porous  and  sufi"used  with  chlorite, 
just  as  in  the  diorites.  The  final  result  is  a  mass  showing  aggregate  polar- 
ization with  a  few  determinable  grains  of  silica  and  carbonates,  and  par- 
ticles of  a  whitish  opaque  substance,  but  nothing  determinable  as  kaolin. 

'  It  has  been  shown  of  late  years  that  the  evidence  afforded  by  fluid  inclusions  needs  to  be  treated 
■with  caution,  for  they  are  reported  as  present  in  all  the  younger  rocks.  No  one,  however,  has  claimed,  so 
far  as  I  am  aware,  that  such  inclusions  are  frequent  in  or  characteristic  of  the  Tertiary  oruptives.  Pro- 
fessor Rosenbusch,  in  liis  "Pliysiog.  der  Gesteine,"  does  not  mention  a  single  observation  of  his  own  on 
fluid  inclusions  in  augite-andesites,  and  cites  only  one  instance  of  such  an  occurrence  noted  by  others. 

If  my  inferences  as  to  the  secondary  nature  of  certain  fluid  inclusions  (p.  79)  are  correct,  a  deduc- 
tion may  need  to  be  made  from  the  number  of  fluid  inclusions,  to  which  a  genetic  significance  can  prop- 
erly be  attributed. 


THE  ROOKS  OF  THE  WASHOE  DISTRICT.  51 

Field  habit. — The  commonest  variety  of  the  east-country  diabase  is  a  fine- 
grained blackish-green  I'ock,  the  most  noticeable  macroscopical  peculiarity  of 
which  is  its  tendency  to  develop  smooth  fissui-e  planes.  Sometimes  these 
planes  are  parallel,  and  of  course  divide  the  rock  into  sheets.  In  other  cases, 
quite  as  common,  they  form  all  sorts  of  angles  with  one  another,  and  divide 
the  rock  into  polyhedral  fragments,  almost  like  large  crystals,  or  into  prisms  of 
various  angles;  but  I  failed  to  find  any  law  governing  the  angular  relations. 
There  can  be  little  question  that  the  cleavages  of  the  rock  have  been 
developed  by  the  dynamical  action  which  has  repeatedly  racked  the  hang- 
ing wall;  but  the  tendency  to  jointing  and  the  planes  of  cleavage  may 
have  been  involved  in  the  original  structure  of  the  rock,  for  the  hammer 
develops  only  the  imperfectly  conchoidal  and  somewhat  rough  surfaces, 
which  other  fine-grained  rocks  show  when  fractured,  and  not  smooth  planes. 
Possibly,  however,  such  might  result  from  a  slow  but  irresistable  pressure. 
The  coai-se-grained  diabases  show  much  less  of  this  jointing,  but  the  fract- 
ure of  both  presents  the  same  appearance  except  in  regard  to  scale — a 
granular  surface  with  fi-equent  larger  lath-like  plagioclases.  In  a  great 
proportion  of  cases  the  feldspars  are  pellucid,  even  when  the  augite  is 
wholly  decomposed;  but  when  the  coarser  rocks  are  so  far  altered  that 
the  feldspars  become  opaque,  the  rock  looks  very  like  diorite,  a  resem- 
blance which  is  greatly  increased  by  the  comparative  absence  of  joints. 
The  diabase  on  the  south  side  of  Ophir  ravine  looks  very  like  a  diorite, 
though  here  the  exposure  is  so  large  that  the  jointing  is  clearly  visible. 
In  many  cases  under  ground  it  is  little  developed,  not  more  so  than  is  fre- 
quently the  case  with  the  diorite.  In  a  few  places,  as  for  example  the 
2,700-foot  level  of  the  Yellow  Jacket,  there  are  limited  occurrences  of  exces- 
sively fine-grained,  closely  laminated  diabase  resembling  slate.  The  diorites 
and  both  the  andesites  show  the  same  phenomenon. 

It  will  be  seen  that  the  andesites  behave  very  differently  in  subterra-- 
nean  and  subaerial  decomposition.  The  behavior  of  the  diabase  in  this 
respect  cannot  be  directly  compared  with  the  later  rocks,  because  the  ex- 
posure in  Ophir  ravine  is  but  little  affected,  and  that  near  the  Ward  is  obscure 
and  almost  wholly  covered  with  wash;  but  the  protection  of  occasional 
masses  of  diabase  from  decomposition  by  accidental  arrangements  of  fissures 


52  GEOLOGY  OP  THE  COMSTOCK  LODE. 

and  clay  seams  can  be  seen  very  perfectly  in  some  of  the  mines,  as  well  as 
extensive  disintegration  of  decomposed  portions,  and  there  can  be  little 
doubt  that  the  behavior  under  erosion  would  be  analogous.  The  pistachio- 
green  so  often  seen  in  the  diorites  and  hornblende-andesites  is  less  common 
in  the  decomposed  diabases,  simply  because  the  prevalent  secondary  mineral 
is  not  epidote  but  chlorite.  The  chlorite  is  sometimes  peculiarly  distrib- 
uted in  blackish,  rounded  spots  on  a  lighter  ground. 

Diagnostic  points. — Dlabasc  Is  Hkcly  to  be  confounded  with  diorite  chiefly 
when  the  feldspars  have  lost  their  transparency.  The  best  indication  macro- 
scopically  is  then  the  lath-like  feldspars,  which  are  rare  in  diorite.  The 
granular  fracture,  though  it  may  be  very  fine-grained,  is  usually  sufficient 
to  separate  it  from  augite-andesite.  Hoi-nblendic  diabases  in  some  cases 
greatly  resemble  hornblende-andesites,  which  are  often  rather  granular;  but 
hornblende  is  not  very  common  or  widely  distributed  in  the  diabase,  and  if 
one  specimen  arouses  a  doubt,  another  can  generally  be  found  near  by 
which  will  set  it  at  rest. 

Younger  diabase. — Tlic  "black  dlkc "  Is  a  fcaturc  which  has  long  been  ob- 
served on  the  CoMSTOCK.  It  extends  horizontally  more  than  a  mile  through 
some  of  the  most  important  mines,  and  occurs  from  near  the  surface  to  the 
lowest  levels  reached.  It  lies  upon  the  foot  wall,  and  is  nowhere  more  than 
a  few  feet  in  thickness.  When  fresh  it  is  of  dark-blue  color  and  a  granular 
texture,  without  the  least  tendency  to  a  porphyritic  structure.  Surfaces 
which  have  been  exposed  only  a  few  hours  turn  to  a  smoky  brown  tint, 
a  peculiarity  shared,  by  no  other  rock  in  the  district. 

Under  the  microscope  it  is  seen  to  be  composed  of  triclinic  feldspar, 
augite,  and  magnetite.  The  feldspars  are  mostly  developed  in  lath-like 
shapes,  and  are  of  very  uniform  size.  They  give  angles  of  extinction  cor- 
responding to  labradorite.  The  augites  are  of  the  usual  color,  but  seldom 
well  developed,  and  to  a  large  extent  occupy  the  interstices  between  the 
feldspars.  The  rock  is  singularly  free  from  inclusions  of  liquid  or  glass; 
indeed,  none  such  have  been  made  out  with  certainty.  The  brownish  tint 
seems  to  arise  from  a  suffusion  of  the  minerals  with  brown  oxide  of  iron, 
and  this  substance  is  very  likely  produced  by  the  oxidation  of  some  chlo- 
ritic  mineral,  of  which,  however,  little  is  visible  under  the  microscope. 


THE  EOOKS  OP  THE  WASHOE  DISTRICT.  53 

Diabasitic  character. — As  tliis  I'ock  IS  wliolly  diflferent  from  the  diabase  of 
the  east  country,  and  is  evidently  younger  than  either  wall  of  the  Lode,  the 
question  naturally  arose  whether  it  might  not  be  a  peculiar  form  of  augite- 
andesite.  This  supposition,  however,  proves  untenable  on  closer  examina- 
tion. The  tendency  of  augite-andesite  is  to  glassy  forms,  and  this  tendency 
could  scarcely  fail  to  be  developed  to  more  than  a  usual  degree,  had  it 
been  injected  into  so  narrow  a  fissure  as  that  which  the  black  dike  must 
have  filled ;  and  any  hypothesis  which  might  be  invented  to  account  for  its 
having  crystallized  much  more  uniformly  and  thoroughly  than  usual  would 
seem  very  forced. 

The  black  dike,  moreover,  thoroughly  resembles  diabases  from  other 
localities,  and  indeed  represents  a  type  of  diabase  which  is  much  more  widely 
distributed  than  the  variety  which  forms  the  east  wall  of  the  Comstock.  The 
rock  from  Orange  Mountain,  New  Jersey,  for  example,  possesses  the  same 
color,  turns  brown  in  the  same  way,  has  the  same  microscopical  characteristics, 
and,  in  short,  is  indistinguishable  from  it  except  by  the  label.  The  analysis 
of  black  dike  is  conclusive  evidence  of  its  diabasitic  character. 

Little  can  be  said  of  the  weathering  of  this  rock  beyond  the  fact  that 
it  passes  into  a  black  clay;  almost  the  only  form  in  which  it  was  observed 
in  the  upper  levels.  To  some  extent  it  has  been  confounded  in  the  Gold 
Hill  mines  with  underlying  black  slates,  with  which,  however,  it  has  exceed- 
ingly little  in  common  except  the  color. 

Had  black  dike  occurred  in  a  fresh  condition  on  the  upper  levels 
former  observers  would  assuredly  have  recognized  its  true  character,  and 
the  east  wall  would  never  have  been  supposed  to  be  of  Tertiary  origin. 


EARLIER    HORNBLENDE-ANDESITE. 

General  character. — Tlic  tlioroughly  fresli  homblendc-andesites  are  macro- 
scopically  dark-bluish  rocks,  showing  porphyritical  crystals  of  hornblende. 
The  feldspars  are  scarcely  perceptible,  except  as  they  express  themselves  in 
the  crystalline  fracture,  on  account  of  their  transparency.  Where  the  horn- 
blendes are  small,  the  appearance  is  consequently  somewhat  basaltic. 

No  base  has  been  recognized  in  the  earlier  hornbleude-andesite  of  the 


54  GEOLOGY  OF  THE  COMSTOGK  LODE. 

District.  The  prevalent  variety  contains  much  augite;  sometimes  even 
more  augite  than  hornblende,  but  no  mica.  There  are  also  micaceous  oc- 
currences, and  these  are  nearly  or  quite  free  from  augite. 

Hornblende. — Tlic  homblendc  is  always  brown  in  the  fresh  rocks,  occa- 
sionally with  a  reddish,  and  often  with  a  greenish,  tinge.  Of  course  it  is 
highly  dichroitic,  and  the  angles  of  extinction  appear'  in  some  cases  to 
exceed  20°.  The  crystal  form  is  the  ordinary  combination  of  prism  and 
clinopinacoid ;  terminal  faces  too,  though  rarer  than  in  augite,  sometimes 
occur.  The  cleavages  are  usually  developed,  though  in  the  freshest  crystals 
they  are  marked  b}'  such  narrow  lines  that  under  a  low  power  they  seem 
absent.  In  one  case  a  clinopinacoidal  cleavage  was  observed.  Twins  are 
very  common.  Glass  inclusions  occur,  generally  as  negative  crystals,  and 
apatites  are  often  inclosed.  Very  rarely  indeed  a  slide  shows  a  particle  or 
fragment  of  hornblende  inclosed  in  another  mineral,  but  as  a  rule  all  the 
hornblende  is  concentrated  in  porphyritical  crystals,  and  does  not  enter  into 
the  groundmass.  I  discovered  only  a  single  very  small  area  in  which  the 
rock  shows  a  large  amount  of  hornblende  distributed  through  the  groundmass 
in  minute  particles;  and  even  in  this  case  the  difference  seems  to  be  one  of 
degree  rather  than  of  kind;  for  the  minute  hornblendes  are  in  large  part 
well  developed  and  appear  to  be  "crystals  of  first  consolidation."  The  black 
border  accompanies  all  the  hornblendes  in  most  of  the  andesites.  Often  it  is 
very  heavy,  and  sometimes  so  encroaches  on  the  crystal  that  little  or  none  of 
the  mineral  appears  in  the  center.  I  have  noticed  no  instances  in  which  black- 
bordered  hornblendes  accompany  crystals  of  the  same  mineral  w*ithout 
black  borders.  In  several  cases  a  double  black  border  is  visible,  the  inner 
one  concentric  with  the  outer,  leaving  a  zone  of  hornblende  between. 
Such  a  case  is  described  nnder  slide  450,  and  shown  in  Fig.  17,  Plate  III. 
I  venture  to  offer  some  speculations  on  this  phenomenon  elsewhere.  The 
black  border  is  readily  soluble  in  chlorhydric  acid,  even  where  the  slide  con- 
tains ilmenite  A  very  few  slides  show  hornblendes  without  black  borders. 
One  such  exception  is  from  the  Sutro  Tunnel  in  a  region  of  intense  sol- 
fataric  activity.     Here  the  hornblendes  are  in  part  very  fresh,  while  the 

'  I  say  appear,  because  it  is  seldom  possible  to  make  absolutely  sure  that  a  crystal  is  cut  exactly 
in  either  of  the  three  principal  zones,  and  a  very  small  obliquity  often  greatly  alters  the  angle  of  extinc- 
tion. 


THE  ROCKS  OF  THE  WASHOE  DISTRICT.  55 

remainder  of  the  rock  is  not.  Cases  occur  on  the  surface  in  which  it  is 
evident  that  the  black  border  has  been  attacked  before  the  hornblende,  and 
this  slide  may  represent  such  an  instance. 

Augite. — The  augites  are  essentially  similar  to  those  of  the  augite-andesites, 
but  it  may  be  mentioned  that  in  one  case  a  pinacoidal  cleavage  was  observed 
which  I  have  never  noticed  in  the  augite  rock.  In  a  slide  from  an  area 
which  I  have  classed  as  hornblende-andesite,  the  augite  also  shows  heavy 
black  borders  like  those  of  the  hornblende.  Augite  is  frequently  present 
in  the  groundmass  in  crippled  crystals  and  irregular  grains,  which  appear 
to  me  referable  to  "secondary  consolidation."  The  jDroportion  of  augite  to 
hornblende  is  always  large  except  in  the  micaceous  andesites,  and,  according 
to  Professor  Rosenbusch,  this  is  common  elsewhere ;  while  in  the  augite- 
andesites  of  the  Washoe  disteict  thei'e  must  be  more  than  one  hundred 
times  as  much  augite  as  hornblende.  I  have  not  always  seen  my  way, 
however,  to  determining  slides  containing  a  decided  excess  of  augite  other- 
wise than  as  hornblende-andesite,  for  such  rocks  occur  in  areas  which  appear 
characteristically  hornblendic.  While  in  such  cases,  which  are  exceptional, 
the  endeavor  has  been  made  to  take  all  the  circumstances  into  considera- 
tion, it  must  be  confessed  that  where  very  augitic  hornblende-andesites  and 
very  hornblendic  augite-andesites  come  together,  the  lines  of  contact  laid 
down  may  be  somewhat  inaccurate,  though  the  error  cannot  be  great;  and 
as  these  conditions  appear  to  prevail  only  along  Cedar  Hill  Canon  it  is  of 
small  importance. 

The  mica  of  the  andesites  gives  the  interference  figure  of  biotite.  It 
is  frequently  black-bordered,  and  the  border  is  usuall}^  deeper  than  that 
around  the  accompanying  hornblende. 

Feldspar. — The  feldspars  of  the  hornblende-andesites  are  nearly  without 
exception  triclinic,  and  of  course  they  can  be  divided  into  porphyritical 
crystals  of  first  consolidation  and  microlites  of  second  consolidation.  As  for 
the  species,  the  porphyritical  crystals  are  either  labradorite  or  anorthite,  and 
the  microlites  either  oligoclase  or  labradorite.  Crystals  giving  anorthite 
angles  of  extinction  have  been  found  in  only  a  few  cases,  and  in  these  I 
suspect  a  mixture  of  anorthite  and  labradorite,  because  while  many  crystals 
seemed  so  placed  that  had  they  been  anorthite  they  must  have  given  angles 


56  GEOLOGY  OF  THE  COMSTOCK  LODE. 

of  extinction  exceeding  those  of  labradorite,  only  a  few  such  sections  gave 
above  32°,  while  many  of  the  remainder  gave  within  a  degree  or  two  of 
31°.  But  I  know  of  no  way  of  absolutely  proving  this  point.  The  feld- 
spars very  often  show  a  zonal  structure.  A  beautiful  case  of  this  kind  is 
mentioned  under  slide  20.  Simply  twinned  feldspars  are  rare,  and  most 
are  poly  synthetic,  according  to  the  albite  law ;  pericline  twinning  is  very 
common,  and  both  of  these  sometimes  appear  in  combination  with  Carlsbad 
twinning.  The  stripes  are  ordinarily  fairly  uniform,  and  of  considerable 
width;  but  sometimes  one  or  both  sets  are  exceedingly  fine,  and  not  uncom- 
monly they  do  not  penetrate  the  crystal,  so  that  one  end  shows  stripes  while 
the  other  does  not.  It  need  scarcely  be  said  that  in  such  cases  the  unstriped 
portion  if  favorably  placed  may  be  proved  to  be  triclinic  by  its  optical 
properties.  The  porphyritical  feldspars  are  usually  developed  in  long  lath- 
like forms.  The  feldspars  contain  inclusions  of  glass  in  almost  every  slide, 
either  as  negative  crystals  or  as  rounded  bodies,  and  these,  when  fresh,  ordi- 
narily carry  bubbles.  Inclusions  of  groundmass  too  are  common,  and 
inclosed  microlites  occur  both  of  apatite  and  of  what  appears  to  be  augite. 
The  latter  are  not  sharply  crystallized,  and  are  generally  fresh,  though  occa- 
sionally accompanied  by  chlorite.  They  are  light  yellow,  and  sometimes 
give  angles  of  extinction  of  above  30°.  I  have  seen  no  fluid  inclusions 
in  such  feldspars  as  seemed  to  be  unaffected  by  decomposition. 

other  minerals. — The  apatitcs  are  usually  colorless,  but  sometimes  brown 
and  dusty.  They  seem  to  be  universally  distributed.  Zircon  occurs  in  only 
one  or  two  slides.  The  iron  ore  is  for  the  most  part  magnetite,  but  occa- 
sionally ilmenite  is  present.  Fig.  19,  Plate  III.,  shows  an  excellent  ilmenite 
section  from  the  highly  augitic  andesite  in  Cedar  Hill  Cauon,  and  the 
application  of  chlorhydric  acid  established  its  presence  with  certainty  in  the 
typical  hornblende-andesite  from  near  the  ComhinaUon  shaft. 

The  groundmass  consists  of  feldspar  microlites  usually  referable  to 
oligoclase,  magnetite,  and  sometimes  microlites  of  augite  Fluidal  structure 
is  common.  Of  course  the  groundmass  must  have  crystallized  in  cooling, 
and  the  question  is  suggested  why  the  glass  inclusions  were  not  devitrified 
at  the  same  time;  but  it  is  evident  that  a  large  part  of  each  porphyritical 
crystal  must  have  formed  after  the  glass  was  inclosed,  leaving  a  residual 


THE  ROOKS  OP  THE  WASHOE  DISTEICT.  57 

magma  of  a  different  composition.  In  only  one  or  two  cases  has  anything 
hke  a  thoroughly  granular  structure  in  the  groundmass  been  observed. 
The  greater  part  of  the  feldspar  microlites  are  generally  well  and  sharply 
developed.  The  same  is  true  in  the  augite-andesites,  and  in  cases  of  extreme 
decomposition  the  shape  of  the  feldspars,  large  and  small,  is  an  important 
point  of  distinction  between  andesites  and  the  older  porphyritic  rocks. 

Field  character. — In  tlio  uiost  important  part  of  the  District  lying  in  the  im- 
mediate neighborhood  of  the  productive  portion  of  the  Lode,  the  hornblende- 
andesite  is  dark  and  fine-grained,  and  contains  only  small  hornblendes,  which 
are  recognizable  as  such  more  often  by  their  brilliant  surfaces  and  evidences 
of  cleavage  than  by  their  crystal  form.  The  rock  breaks  easily  under  the 
hammer  with  a  somewhat  conchoidal  fracture,  and  its  luster  is  more  or  less 
glassy.  The  hornblende-andesites  which  occur  south  of  Gold  Hill  are  much 
more  porphyritic,  and  the  hornblendes  are  unusually  well  developed. 
Crystals  of  an  inch  and  a  half  in  length  are  common,  and  one  decomposed 
crystal  fully  four  inches  long  was  observed.  In  none  of  the  varieties  are  the 
feldspars  visible  when  fresh  except  on  minute  examination,  simply  because 
they  are  transparent,  and  the  dark  color  is  therefore  due  to  the  bisilicates 
and  magnetite.  Columnar  structui'e  is  occasionally  developed  all  over  the 
district,  but  in  no  great  perfection. 

Weathering. — Ordinarily  the  hornblende-andesite  appears  to  possess  little 
or  no  structui'e  in  mass,  while  under  the  action  of  the  atmosphere  it  develops 
considerable  fissility  in  certain  directions,  so  that  some  croppings  present 
almost  the  appearance  of  upturned  beds  of  sedimentary  rocks  with  parallel 
partings  at  a  distance  of  one  or  two  inches.  That  the  fissile  tendency  does  not 
extend  to  an  indefinite  lamination  is  evident  from  the  behavior  of  the  sherds. 
These  do  not  continue  to  part  parallel  to  their  more  extended  surfaces,  but 
are  gradually  rounded  by  the  action  of  frost.  By  this  agency  conchoidal 
fragments  are  separated  from  the  corners  and  edges  of  the  loos6  blocks, 
and  when  it  is  considered  through  how  short  a  distance  the  action  of  the 
frost  can  extend,  the  display  of  force  is  quite  astonishing.  Conchoidal  chips 
of  three  or  four  pounds  in  weight  are  often  found  at  a  distance  of  two  or 
three  feet  from  the  block  on  which  they  fit.  Large  masses  of  hornblende- 
andesite  breccia  also  occur,  though  this  form  is  not  so  common  as  with  the 


58  GEOLOGY  OF  THE  COMSTOCK  LODE. 

augite-andesites.  Of  coiirse,  neither  columnar  structure  nor  fissilitj^,  both 
of  which  are  probably  to  be  regarded  as  results  of  tension  from  cooling, 
are  developed  in  the  comparatively  porous  breccias,  for  the  fragments  of 
unfused  rock  in  breccia  act  like  the  chamotte  in  a  fire-brick  in  preventing 
density  of  structure. 

Decomposition. — The  Weathering  of  the  hornblende-andesite  seems  to  differ 
in  its  nature,-  as  it  takes  place  in  direct  contact  with  the  air  or  under  ground. 
Croppings  of  the  rock  which  on  being  broken  prove  internally  fresh,  are  com- 
monly coated  with  a  very  thin,  deep-red  or  brown  scale  and,  to  judge  by 
fragments  found  in  the  immediate  neighborhood  of  such  croppings,  the 
change  seems  to  consist  mainly  in  disintegration  by  frost  and  in  peroxida- 
tion of  the  iron.  Under  ground,  on  the  other  hand,  decomposition  appears  to 
extend  into  the  body  of  the  rock.  One  of  the  first  minerals  to  be  afi"ected 
is  the  feldspar,  which  loses  its  transparency  and  becomes  a  dead  white. 
This  totally  alters  the  appearance  of  the  rock,  which  becomes  a  light-gray 
porphyry,  instead  of  a  dark-bluish  and  basaltic-looking  mass.  Every  varia- 
tion in  coarseness  of  grain  also  becomes  apparent.  The  feldspars  lose 
their  transparency  when  only  a  very  minute  portion  of  their  substance 
(certainly  less  than  one  per  cent.)  is  altered.  The  next  stage  of  decompo- 
sition is  the  formation  of  chlorite  from  the  bisilicates,  which  soon  diffuses 
itself  tln-ough  the  groundmass  and  the  feldspars.  The  chlorite  is  further 
frequently  decomposed  into  calcite  and  epidote  without  any  special  change 
in  the  appearance  of  the  rock.  All  these  changes  tend  to  diminish  the 
sharp  definition  of  the  porphyritical  crystals  and  give  the  mass  the  look 
rather  of  an  olde)'  dioritic  porphyry  than  of  a  volcanic  rock.  It  is  easy  to 
suggest  plausible  explanations  for  the  different  behavior  of  the  andesite 
above  ground  and  beneath  the  surface.  The  presence  under  ground  of 
water  holding  carbonic  acid  in  solution  is  perhaps  sufficient  to  account  for 
the  formation  of  calcite  in  the  feldspars,  and  the  strong  oxidizing  action  on 
the  surface  may  well  explain  the  direct  formation  of  ferric  oxide  in  the 
exposed  rocks.  When  the  andesites  are  not  in  the  condition  of  breccia  the 
subterranean  decomposition  is  commonly  accompanied  by  a  softening  or 
partial  disintegration  of  the  mass,  though  in  some  cases,  as  at  the  South 
Twin  Peak,  rock  not  brecciated  preserves  great  consistency,  possibly  from 


THE  ROCKS  OF  THE  WASHOE  DISTRICT.  59 

an  originally  porous  texture.  The  breccias  remain  hard  and  tough  until 
every  mineral  has  been  subjected  to  complete  alteration.  There  is  much 
evidence  and  every  analogy  to  show  that  this  decomposition  proceeds  from 
external  surfaces,  cracks,  and  fissures  toward  the  centers  of  blocks  or  masses. 
Very  frequently  where  cuts  have  exposed  altered  rocks,  blocks  of  small  size 
may  be  seen,  which  consist  of  concentric  shells  of  loose  decomposed  rock- 
substance,  and  still  contain  kernels  of  fresh  andesite.  The  size  of  the  blocks 
is,  of  course,  a  matter  of  accident,  and  sometimes  extensive  masses  decom- 
pose only  from  their  external  surfaces..  When  this  is  the  case  erosion 
often  acts  more  rapidly  than  decomposition  and,  as  the  decomposed  rock 
is  comparatively  soft,  masses  of  the  fresh  andesite  are  frequently  left  standing- 
above  the  general  level.  The  fresh  rock  thus  exposed  has  the  appearance 
of  a  cropping  of  a  younger  eruption  penetrating  and  overlying  an  older  and 
different  one;  and  this  appearance  is  heightened  by  the  weathering  of  the 
pseudo  cropping  which,  as  already  explained,  results  in  a  mass  of  reddish- 
brown  fragments  quite  unlike  the  product  of  alteration  beneath  the  surface. 
The  andesite  which  had  decomposed  under  ground  used  to  be  regarded  as 
propylite,  but  careful  examination  of  exposed  masses  of  andesite  such  as 
those  described,  shows  that  a  transition  into  the  propylitic  form  may  alwa)^s 
be  followed  out  at  their  base.  As  the  course  of  the  decomposition  is  depend- 
ent on  the  presence  of  accidental  fissures  and,  no  doubt,  on  the  texture  of 
the  rock,  the  form  of  the  residual  masses  of  undecomposed  andesite  is  fan- 
tastically various,  sometimes  resembling  dikes,  again  assuming  the  shape  of 
domes  and  cones. 

Distinctive  characteristics. — Hombleude-andesites  are  distinguishable  from  the 
augite-andesites  when  fresh  by  the  presence  of  abundant  porphyritic  horn- 
blende crystals  and  by  the  luster,  which  in  the  augitic  rocks  is  resinous. 
From  the  porphyritic  diorites  they  are  distinguishable  macroscopically  by 
a  lack  of  the  granular  structure,  which  the  older  rock  commonly  shows. 
In  the  propylitic  stage  of  decomposition  the  three  rocks  are  almost  indis- 
tinguishable. 

Speculation  on  "black  border." — Somc  of  tlic  Washoe  audesltes  sccm  capable  of 
throwing  light  on  the  conditions  under  which  the  black  border  forms  about 
hornblende  crystals.     In  slides  from  different  parts  of  the  District  two  can- 


60  GEOLOGY  OP  THE  COMSTOCK  LOBE. 

centric  belts  of  magnetite  have  been  observed,  separated  by  hornblende-sub- 
stance. Much  the  finest  instance  is  illustrated  in  Fig-.  17,  Plate  III.  There 
can  be  little  doubt  from  direct  observation  on  modern  lavas  that  porphj'-- 
ritical  crystals  are  formed  prior  to  eruption,  and  a  tolerably  large  and  ver}^ 
sharply  defined  specimen,  like  that  shown  in  the  drawing,  is  not  likely  to  be 
an  exception.  At  some  time  after  it  ceased  to  grow  this  crystal  was  broken; 
but  the  external  black  border  was  formed  at  a  still  later  period,  for  it  is  as 
heavy  on  the  fractured  surface  as  on  the  crj^stal  faces.  It  is  difficult  to  imagine 
a  mass  of  melted  lava  in  a  state  of  agitation  sufficiently  violent  to  break  crys- 
tals suspended  in  the  fluid  magma,  except  during  an  actual  eruption,  and  it 
may  be  inferred  with  some  probability  tliat  this  was  fractured  in  its  passage  to 
the  surface.  If  so,  the  external  black  border  was  probably  formed  as  the  rock 
cooled  after  eruption.  The  inner  belt  of  magnetite,  on  the  other  hand,  indi- 
cates a  che(;k  in  the  growth  of  the  crystal,  and  must  have  bSen  formed  long 
before  ejection^.  But  it  is  impossible  to  suppose  the  temperature  to  vary  greatly 
in  melted  rock-masses,  at  the  depth  below  the  surface  at  which  it  is  believed 
that  eruptions  originate.  The  pressure  upon  subterranean  fluid  masses, 
however,  probably  varies  within  very  wide  limits,  and  it  is  well  known  that 
changes  in  pressure  produce  effects  closely  analogous  to  those  caused  by 
variations  in  temperature.  It  seems  on  the  whole,  therefore,  most  likely 
that  this  hornblende  errew  to  the  limits  of  the  inner  black  border  under 
conditions  which  were  uniform,  or  perhaps  varied  uniformly;  that  a  sudden 
change  in  pressure  equivalent  to  a  diminution  of  temperature  induced  the 
secretion  of  magnetite;  that  the  conditions  for  hornblende  secretion  were 
then  reestablished,  and  continued  till  the  time  of  the  eruption,  when  the 
crystal  was  fractured,  and  became  surrounded  by  a  second  border  during 
the  cooling  process.  Other  large  hornblendes  in  the  same  slide  also  have 
double  black  borders,  though  less  symmetrically  developed,  but  the  smaller 
hornblendes,  though  also  of  considerable  size,  and  manifestly  "crystals  of 
first  consolidation,"  show  only  a  single  external  belt  of  magnetite,  as  if  their 
formation  had  begun  only  after  the  temporary  change  in  pressure.  If  the 
hypothetical  history  suggested  is  correct,  it  is  pi'obable  that  hornblende  only 
forms  under  conditions  of  pressure  which  have  not  yet  been  reproduced  in 
the  attempt  to  crystallize  the  mineral  artificially,  and  the  comparative  rarity 


THE  ROCKS  OF  THE  WASHOE  DISTRICT.  61 

of  the  black  border  about  augite  may  indicate  that  this  mineral  is  less  influ- 
enced by  differences  of  pressure.  The  basis  of  the  whole  speculation  is, 
however,  exceedingly  slender. 

Discussion  of  a  zonal  piagiociase. — Zonal  structuro  is  exceedingly  common  in  the 
feldspars  of  nearly  all  the  rocks  of  Washoe,  and  not  infrequently  there  is  a 
nearly  uniform  and  progressive  change  in  the  optical  properties  from  the  cen- 
ter of  the  crystal  towards  the  periphery  without  demarkation  into  zones.  Of 
course  such  a  feldspar  may  be  regarded  as  consisting  of  an  indefinite  number 
of  zones,  but  while  ordinary  zonal  crystals  show  recurrent  layers  these  do 
not. 

A  remarkable  instance  of  zonal  structure  occurs  in  slide  20  from  the 
North  Twin  Peak.  It  is  illustrated  in  Fig.  13,  Plate  III.  This  feldspar  is 
probably  a  labradorite  cut  on  a  plane  at  right  angles  to  the  brachypinacoid. 
The  outer  edge  and  the  interior  kernel  extinguish  light  almost  simulta- 
neously when  the  cleavage  plane  makes  an  angle  of  about  14°  with  the 
principal  Nicol  section.  The  intermediate  belt,  on  the  contrary,  extin- 
guishes at  an  angle  of  only  5°,  though  in  the  same  direction  as  the  outer 
and  inner  portions.  The  fine  stripes  are  blackest  at  an  angle  of  about  14°, 
with  an  opposite  inclination,  but  they  show  no  zonal  structure  extinguish- 
ing light  at  the  same  angle  throughout  their  entire  length.  The  persistence 
of  these  stripes  throughout  the  crystal  seems  to  prove  its  crystallographic 
unity,  which  is  further  confirmed  by  the  ^parallelism  of  the  zonal  limits. 
The  section  also  shows  very  well  the  alteration  in  form  of  the  feldspar 
during  growth,  as  well  as  the  identity  of  the  zonal  inclusions  with  the 
groundmass,  there  being  a  connection  through  an  opening  on  one  side. 

The  variation  in  the  position  of  the  optical  axes  of  different  portions  of 
a  crystal,  the  effects  of  which  are  seen  in  zonal  structure,  must  be  due  to 
differences  in  crystallographic  orientation,  or  in  tension,  or  in  chemical 
composition.^  Checks  in  the  growth  of  a  crystal  may  produce  demarka- 
tions  such  as  are  shown  in  Fig.  1 7,  Plate  III.,  and  described  on  p.  60,  but 
so  long  as  composition,  tension,  and  orientation  are  the  same,  the  position 
of  the  optical  axes  must  be  constant.  In  the  feldspar  under  discussion  the 
orientation  of  the  zones  cannot  be  different,  and  variations  in  tension  would 

'  Cf.  Min^ralogie  Micrograph,  by  Fouc[u6  &  L6yy,  pp.  36  and  130. 


62  GEOLOGY  OF  THE  COMSTOCK  LODE. 

be  visible  in  the  narrow  lamellae  as  well  as  in  the  broad  ones.  The  intru- 
sive groundraass,  too,  is  scarcely  compatible  with  the  supposition  of  variable 
tension,  and  the  zonal  structure  in  this  case  must  be  due  to  modifications 
in  chemical  composition.  This  may  vary  in  two  ways;  there  may  be  a  sub- 
stitution of  isomorphous  elements  without  disturbance  of  the  characteristic 
"oxygen  ratio"  (atomic  ratio)  of  the  mineral  species,  or  this  ratio  may  be 
modified  in  the  sense  of  Professor  Tschermak's  feldspar  theory.  Granting 
the  accuracy,  or  even  the  approximate  accuracy,  of  Messrs.  Fouqu(^  & 
Levy's  discussion  of  the  optical  properties  of  labradorite^  and  other  feld- 
spars, the  first  supposition  is  impossible  in  the  present  case;  for  if,  at  the 
position  indicated  by  the  angle  of  extinction  of  the  thin  lamellse  and  two  of 
the  zones,  this  angle  may  vary  10°,  the  distinction  of  difii'erent  species  by 
this  property  is  illusory.  Indeed  the  extinctions  are  consistent  with  the  sup- 
position that  the  intermediate  belt  is  oligoclase,  an  hypothesis,  however, 
with  which  the  crystallographic  unity  of  the  section  is  incompatible.  I  am 
therefore  forced  to  the  supposition  that  the  intermediate  belt  answers  to  a 
variety  of  feldspar  of  a  different  oxygen  ratio  from  labradorite,  but  crystal- 
lizing in  this  mixture  in  the  same  form.^  The  same  explanation  seems  to 
me  indicated  in  most  zonally -built  plagioclases,  and  in  those  which  display 
progressive  divergence  of  the  optical  axes. 


AUGITE-ANDESITE. 

General  character. — Thc  augite-andcsitcs  prcseut  the  closest  parallellism  to 
thehornblende-andesites;  the  resemblance  being  far  closer  than  that  which 
exists,  for  example,  between  the  diorite  and  the  diabase.  But  for  the  fact 
that  they  clearly  belong  to  different  eruptions  it  would  seem  more  appropriate 
to  regard  the  two  rocks  as  varieties  rather  than  as  independent  species.  In 
the  Washoe  district  the  porphyritic  augites  are  rarely  macroscopically 
noticeable,  but  their  effect  is  perceptible  in  a  certain  resinous  luster.  While 
the  color  of  the  earlier  hornblende-andesite  in  a  fresh  condition  is  commonly 

iL.  c,  p.  253. 

°The  influence  of  salts  of  analogous  properties,  when  mixed,  in  modifying  the  resultant  crystal 
form  is  ■well-known. 


THE  ROCKS  OF  THE  WASHOE  DISTRICT.  G3 

a  blue-gray,  not  unlike  "teinte  neutre,"  the  augite-andesites  are  generally 
a  much  deeper,  somewhat  brownish-blue.  Certain  glassy  augite-andesites 
strongly  resemble  the  glassy  hornblende-andesites,  while  another  variety  is 
pinkish-gray,  and  bears  no  superficial  resemblance  to  anything  else  in  the 
DiSTKiCT.  Some  gray  vesicular  modifications  have  a  basaltic  look.  The 
crystalline  augite-andesites  greatly  predominate  over  the  glassy  ones. 
Hornblendes  occur  in  a  majority  of  specimens,  but  in  very  small  numbers 
as  compared  with  the  augites,  probably  not  one  per  cent.,  while  mica  is  met 
with  only  often  enough  to  justify  the  assertion  of  its  occurrence. 

Augite. — The  augite  is  of  precisely  the  same  character  as  that  of  the 
hornblende-andesites.  Its  color  is  always  a  more  or  less  brownisli-yellow, 
which  varies  somewhat  in  shade  but  not  in  character,  and  is  very  like  that 
of  bamboo.  I  have  not  observed  a  single  case  of  pinacoidal  cleavage,  while 
there  is  a  decided  tendency  to  the  suppression  of  one  of  the  prismatic  cleav- 
ages. In  some  specimens  the  proportion  of  augite  is  small,  and  the  crystals 
are  then  very  well  developed.  In  other  cases  they  are  very  numerous  and 
occur  in  groups  in  which,  owing  apparently  to  interference,  the  crystallo- 
graphic  outlines  are  imperfectly  developed.  They  frequently  contain  glass 
inclusions,  which  sometimes  assume  the  form  of  negative  crystals,  and 
sometimes  spheroidal  shapes;  but  embedded  microlites  of  other  minerals 
are  rare.  Besides  the  porphyritical  crystals,  the  augite  often  appears  to 
form  a  portion  of  the  groundmass,  and  microlites  of  it  are  common  in  the 
feldspars.  In  one  rock,  which  has  been  classified  as  a  hornblende-ande- 
site,  an  augite  was  noted  piercing  an  ilmenite.  These  facts  point  to  a  very 
wide  range  of  time  for  the  crystallization  of  the  augite,  which  would  seem 
to  have  been  among  the  first,  and  among  the  last,  minerals  to  assume  a 
crystalline  form.  This  is  a  strong  contrast  to  the  occurrence  of  hornblende, 
but  in  conformity  with  the  results  of  experiment,  for,  as  is  well  known, 
augitehasbeen  artificially  reproduced  under  a  variety  of  conditions;  whereas, 
so  far  as  I  am  aware,  the  eff"orts  to  reproduce  hornblende  have  hitherto 
proved  unsuccessful.  The  augites  very  exceptionally  show  a  trace  of  the 
black  border,  so  commonly  accompanying  hornblende. 

Other  minerals. — The  homblendo  is  precisely  similar  to  that  of  the  horn- 
blende-andesites.    It  usually  occurs  in  minute  crystals,  with  heavy  black 


64  GEOLOGY  OF  THE  COMSTOCK  LODE. 

borders;  but  in  one  very  glassy  rock  it  lacks  this  accompaniment.  The 
feldspars  are  also  entirely  similar  to  those  in  the  preceding  rock.  Anorthite 
has  been  identified  in  a  few  slides  among  the  larger  crystals,  but  in  most 
cases  the  maximum  angles  of  extinction  correspond  to  labradorite.  The 
microlitic  feldspars  appear  generally  to  be  oligoclase.  The  iron  ore  is  com- 
monly magnetite,  but  in  a  few  cases  chai'acteristic  ilmenite  sections  have 
been  observed.  Apatite  is  invariably  present,  very  frequently  as  brown  or 
dusty  crystals.  There  is  no  inconsistency  between  the  presence  of  the  brown 
apatite  and  the  colorless  variety,  which  often  occur  in  profusion  in  the  same 
slide;  but  the  brown  crystals  seem  rarely  to  assume  the  acicular  form  which 
so  generally  jjrevails  among  colorless  apatites.  I  have  not  observed  a  single 
zircon,  nor  anything  which  can  be  set  down  with  certainty  as  titanite.  The 
groundmass  of  the  augite-andesites  is  usually  microlitic,  though  in  one  or 
two  cases  granular  structure  has  been  noted.  It  is  very  frequently  the 
case  that  the  microlites  of  feldspar  are  excessively  minute,  and  with  lower 
objectives  the  groundmass  then  gives  the  impression  of  felt.  This  is  an 
appearance  which  the  hornblende-andesites  seldom  present.  The  microlites 
are  often  so  arranged  as  to  produce  the  effect  called  fluidal  structure. 

Field  character. — Tlio  Ordinary  variety  of  augite-andesite  in  a  fresh  condition 
is  dark  blue,  or  brownish-blue,  in  color,  resinous  in  luster,  and  has  a  rough 
fracture.  The  comparatively  fine-grained  varieties  often  show  the  lighter 
colors  and  smoother  fi-actures  common  in  hornblende-andesites,  and  when 
the  rock  is  at  the  same  time  somewhat  hornblendic  it  is  readily  confounded 
with  hornblende-andesite.  Sometimes,  when  the  feldspars  are  unusually 
developed  and  the  fracture  is  excessively  rough,  the  rock  might  be  mistaken 
for  trachyte ;  but  the  absence  of  mica,  the  rarity  of  the  hornblendes,  and  the 
predominance  of  triclinic  feldspars  are  generally  sufficient  to  distinguish 
it.  In  a  few  instances  the  augite-andesites  are  very  granular  and  coarse- 
grained, and  when  slightly  decomposed  do  not  greatly  differ  from  some  dio- 
rites  in  appearance,  but  the  likeness  is  superficial.  An  imperfect  columnar 
structure  is  occasionally  met  with,  but  is  not  characteristic  of  the  rock. 
Breccia  is  exceedingly  common,  and  is  sometimes  tufaceous. 

Decomposition  and  weathering. — As  is  the  case  with  the  homblonde-andesites, 
when  the  rock  is  directly  exposed  to  the  action  of  the  atmosphere  the  process 


THE  EOCKS  OF  THE  WASHOE  DISTRICT.  05 

of  decomposition  is  very  different  from  that  which  it  undergoes  when  buried 
beneath  the  surface.  Croppings  of  the  fresh  rock  rarely  exhibit  the  tendency 
to  divide  into  parallel  plates  so  characteristic  of  the  other  andesite.  The  want 
of  homogeneity  in  structure  displays  itself  in  a  different  but  very  interesting 
manner.  Under  the  action  of  the  weather  it  frequently  becomes  apparent 
that  large  masses  of  augite-andesite  are  composed  of  thin  beds  of  various 
character.  Some  of  these  yield  to  weathering  much  more  rapidly  than 
others,  and  the  exposed  face  becomes  indented  with  closely  set  parallel 
grooves,  such  as  are  often  observed  in  finely  laminated  sedimentary  rocks. 
There  is,  however,  no  perceptible  tendency  to  the  development  of  cracks  in 
the  directions  indicated  by  these  grooves.  The  most  natural  explanation  of 
this  structure  would  seem  to  be  that  they  represent  rapidly  succeeding  flows 
of  the  melted  rock,  but  it  is  hard  to  see  in  that  case  why  differences  of  ten- 
sion do  not  lead  to  the  development  of  fissures.  Other  masses  show  an 
analogous  but  different  behavior  in  the  development  of  grooves  of  sinuous 
form,  which  cross  each  other  at  considerable  angles,  and  give  the  surface 
somewhat  the  appearance  of  an  irregular  pavement.  If  this  structure  were 
found  only  upon  opposite  surfaces  of  blocks;  it  might  be  interpreted  as  an 
expression  of  a  tendency  to  separate  into  columns ;  but  when  it  occurs  at 
all,  it  is  found  equally  on  all  the  faces  exposed.  It  appears  to  me  that 
solidification  must  have  set  in  from  numerous  centers  distributed  through  the 
rock,  giving  it  a  coarse  pseudo-spherolitic  structure,  and  that  the  grooves 
must  represent  a  slight  difference  in  chemical  composition  in  that  portion  of 
the  lava  which  was  the  last  to  solidify.  Whatever  may  be  the  cause  of  the 
appearance,  it  is  highly  characteristic  of  the  rock  in  this  District.  A  good 
example  appears  in  the  foreground  of  the  frontispiece. 

When  fresh  augite-andesite  is  exposed  to  the  air,  it  soon  becomes  coated 
with  a  yellowish-white  product  of  decomposition.  This  is  gradually  con- 
verted into  a  bright  reddish-brown  substance,  no  doubt  largely  ferric  oxide, 
the  surface  at  the  same  time  growing  rough.  In  many  cases  this  color  is 
succeeded  later  by  a  pitchy  black.  The  rate  of  change  is  by  no  means 
slow,  and  in  some  of  the  railroad  cuts,  made  a  dozen  years  since,  decom- 
position has  penetrated  the  rock  for  about  a  quarter  of  an  inch.  There  is 
reason  to  suppose  that  after  the  rock  has  turned  black  the  rate  of  change 

5   0  L 


66  GEOLOGY  OF  THE  COMSTOCK  LODE. 

is  greatly  decreased.  Whilfi  the  changes  in  direct  contact  with  the  air  are 
markedly  different  from  tliose  which  take  place  in  hornblende-andesite,  the 
process  of  decomposition  under  ground  seems  to  be  identical  in  the  two 
rocks;  nor  are  the  products  of  decomposition  distinguishable  after  the  pro- 
pylitic  stage  has  been  reached.  As  is  the  case  with  the  hornblende-andesites, 
too,  solid  augite-andesite  disintegrates,  while  brecciated  masses  retain  their 
consistency,  and  are  consequently  exposed  as  bold  croppings  by  the  erosion 
of  adjoining  disintegrated  rocks.  I  do  not  know  of  any  cases  of  unbrec- 
ciated  augite-andesite  retaining  its  consistency  in  spite  of  considerable 
decomposition,  as  the  hornblende-andesite  of  the  South  Twin  Peak  has 
done. 


LA.TER   HORNBLENDE-ANDESITE. 

General  character. — This  Tock,  most  of  whicli  lias  hitherto  been  regarded 
as  ti'achyte,  varies  greatly  in  appearance  in  di£ferent  parts  of  the  field.  The 
more  trachytic  varieties,  such  as  those  of  the  quarries  a  couple  of  thousand 
feet  northeast  of  Sutro  shaft  No.  Ill ,  are  purplish  or  reddish  soft  rocks, 
loose  in  structure,  and  thickly  studded  with  large  feldspar  crystals,  horn- 
blendes, and  flakes  of  mica.  Near  the  Utah  mine  the  color  is  gray,  and 
the  texture  firmer  and  finer-grained,  while  further  north  the  rock  is  dense, 
black,  and  glassy.     It  also  occurs  largely  as  tufa. 

Fe-Mg  silicates. — All  the  youuger  hornblende-andesite  contains  mica,  though 
in  some  cases  the  amount  of  this  mineral  in  comparison  with  the  bisilicates 
is  small.  Hornblende,  too,  is  always  present,  and  augite  generally  forms  a 
subordinate  constituent.  The  feldspars  are  of  course  triclinic,  and  no  deter- 
minable sanidin  has  been  detected.  Much  of  the  rock  is  thoroughly  crystal- 
line excepting  inclusions,  but  the  extent  of  the  occurrences  showing  a  glassy 
base  is  considerable.  The  hornblende  is  entirely  similar  to  that  of  the  older 
andesite,  but  there  seems  to  be  a  relation  between  the  physical  structure  of  the 
rock  and  the  development  of  black  border.  In  the  coarser,  more  trachytic- 
looking  masses,  the  black  border  of  both  hornblende  and  mica  almost  wholly 
replaces  the  original  mineral,  as  maybe  seen  to  some  extent  in  Fig.  32,  Plate  V. 
In  the  dense  glassy  rocks,  on  the  other  hand,  the  border  is  narrow,  or  alto- 


THE  ROCKS  OF  THE  WASHOE  DISTRICT.  67 

gether  wanting.  The  mica  seems  to  be  biotite  in  most  cases,  but  in  two  or 
three  localities  cleavage  scales  give  an  unmistakably  biaxial  interference 
figure.  It  is  as  uniformly  surrounded  by  a  border  of  magnetite  as  the  horn- 
blende. The  augite  presents  no  peculiarities  in  structure.  The  amount  of 
this  mineral  is  commonly  inversely  as  the  quantity  of  mica.  Magnetite  is 
remarkable  only  for  its  abundance,  and  nothing  which  could  be  pronounced 
titanic  iron  was  noticed.  Apatites  are  rarer  than  in  the  older  volcanic  rocks. 
Feldspars. — Almost  all  the  large  porphyritic  feldspars  show  abundant 
striations,  even  under  the  lens,  and  few  large  crystals  appear  to  lack  them 
under  the  microscope.  Many  feldspars  which  do  not  show  polysynthetic 
structure  under  an  objective  of  low  magnifying  power,  show  striae  under 
higher  powers.  Many  of  the  feldspars  show  zonal  structure  comparable  with 
that  discussed  on  page  fil  and  illustrated  in  Fig.  13,  Plate  III.  The  large 
feldspars  are  manifestly  crystals  of  first  consolidation,  while  the  groundmass 
is  in  great  part  made  up  of  microlitic  feldspars.  While  the  large  crystals  com- 
monly give  angles  of  extinction  indicating  labradorite,  the  microlites  appear 
to  be  chiefly  oligoclase.  There  are  also  among  the  larger  feldspars  a 
comparatively  small  number  of  Carlsbad  twins,  and  simple  crystals  which 
might  be  regarded  as  sanidin  if  no  further  test  were  applied ;  but  none  such 
which  were  cut  in  the  determinable  zones,  gave  angles  of  extinction  ajDpro- 
priate  to  orthoclase.  As  some  of  the  possible  sanidins  were  not  so  oriented 
as  to  make  optical  determinations  practicable,  I  submitted  a  specimen  of  the 
most  trachy tic-looking  rock  in  the  district  to  Dr.  George  W.  Hawes,'  curator 
of  the  National  Museum,  for  separation  by  Thoulet's  method.  The  speci- 
men sent  was  from  a  quarry  2,000  feet  east  of  the  Occidental  mill,  E  5,  and 
was  in  all  respects  identical  with  that  described  by  Professor  Zirkel  under 
slide  283.  The  following  details  are  taken  from  Dr.  Hawes'  report  on  this 
rock : 

Feldspars  determined  by  Thoulet's  method. Tlie      SpecimCU     WaS     pulvCnZed     tO     SUcll 

an  extent  that  it  would  pass  through  a  sieve  containing  three  meshes  to  the 
millimeter;  and  from  this  mass  of  grains  the  dust  that  would  not  settle  was 
separated  by  elutriation.     As  the  special  object  in  view  was  to  determine 

•  While  this  report  was  going  through  the  press  Dr.  Hawes  died  (June  22^  leaving  a  vacancy  in 
the  ranks  of  American  geologists  which  it  will  he  hard  to  fill,  as  well  as  a  deep  personal  regret  in  the 
hearts  of  all  who  knew  him,  however  slightly. 


68  GEOLOGY  OF  THE  COMSTOCK  LODE. 

the  species  of  feldspar,  the  mass  of  grains  was  first  placed  in  a  solution  of  the 
double  iodide  of  potassium  and  mercury,  which  possessed  a  specific  gravity 
of  2.95.  A  portion  of  the  substance  immediatel}^  fell  to  the  bottom.  When 
examined  with  the  microscope  this  was  found  to  consist  of  hornblende, 
augite,  mica,  and  ii'on  oxide.  The  specific  gravity  of  the  fluid  was  then 
diminished  to  2.85,  when  a  small  portion  settled  out.  The  precipitate  was 
found  under  the  microscope  to  consist  of  composite  grains  including  por- 
tions of  one  of  the  previously  mentioned  minerals.  At  the  specific  gravity 
2.75  only  a  few  grains  of  the  same  character  fell  down,  and  these  were 
more  largely  feldspathic. 

On  reducing  the  fluid  to  2.70,  a  large  amount  of  clear  white  grains 
fell  from  the  fluid.  At  2.68  another  large  portion  was  precipitated,  and 
these  precipitates  when  examined  under  the'  microscope  proved  to  be  com- 
posed entirely  of  grains  of  feldspar.  On  reducing  the  specific  gravity  to 
2.fi7  very  little  fell  down,  and  this  was  of  a  red  color,  and  consisted  mostly 
of  grains  containing  clear  feldspar,  together  with  portions  of  the  ground- 
mass.  Subsequent  reductions  of  the  specific  gravity  caused  the  remaining 
substances  to  fall  to  the  bottom  in  successive  portions,  and  when  the  fluid 
had  reached  the  specific  gravity  of  2.61,  only  a  very  small  amount  of  ma- 
terial floated.  This  examined  under  the  microscope  was  found  to  consist 
entirely  of  groundmass.  There  appeared  to  be  no  portion  of  the  glassy 
feldspar  crystals  in  any  of  the  substances  which  had  a  specific  gravity 
below  2.65.  As  the  amount  of  rock  which  will  float  at  any  specific  gravity 
which  approaches  that  of  orthoclase  is  very  small,  it  would  seem  that 
under  no  circumstances  could  this  feldspar  be  considered  as  a  preponder- 
ating species,  and  that,  if  present  at  all,  it  must  be  in  very  small  amount. 

Mr.  F.  P.  Dewey,  at  Dr  Hawes'  request,  analyzed  the  feldspar  which 
fell  when  the  specific  gravity  of  the  solution  was  2.70  and  found  its  oxygen 
ratio  1:2.89:7.95.  This  I  find  would  correspond  to  a  mixture  of  39  per 
cent,  labradorite  and  61  per  cent,  oligoclase,  supposing  these  the  only  feld- 
spars present.  He  also  analyzed  the  portion  which  fell  at  a  specific  gravity 
of  2.68  and  found  the  oxygen  ratio  1:2.96:8.69,  corresponding  to  12  per 
cent.  labradorite,and  88  per  cent,  of  oligoclase.  The  entire  feldspar  analyzed 
was  3 1  per  cent,  of  the  rock,  or  8  per  cent,  labradorite  and  23  per  cent,  oligo- 


THE  EOCKS  OF  THE  WASHOE  DISTRICT.  '  69 

clase,  on  the  supposition  of  a  mere  mixture  of  species.  It  appears  to  me 
more  probable,  however,  from  the  character  of  the  zonal  plagioclases,  that 
many  of  the  feldspars  are  not  chemically  referable  to  either  species. 

The  results  of  the  application  of  Thoulet's  method  agree  excellently 
with  those  of  the  microscopical  examination,  and  together  render  it  impossi- 
ble to  classify  this  rock  otherwise  than  as  a  hornblende-andesite,  in  spite  of 
a  macroscopical  ajDpearance  exceedingly  like  ordinary  varieties  of  trachyte, 
and  very  dissimilar  to  common  andesite. 

Remarkable  glass  inclusion  in  feldspar. The    fcldsparS    COUtalu    glaSS    incluslonS  iu 

all  the  slides  of  this  rock,  but  these  are  most  abundant  to  the  north  of  the 
Utah.  In  the  quarry  close  to  the  hoisting  works  of  that  mine  some  of  these 
inclusions  are  of  a  peculiar  character,  forming  negative  feldsjiar  crystals  of 
a  more  or  less  perfect  shape.  These  were  mentioned  by  Professor  Zirkel 
with  admiration.  No  such  fine  example  occurs  in  my  slides  as  in  that  de- 
scribed by  him,  and  in  his  slide  number  284  there  is  but  one  which  can 
have  furnished  his  description.  This  is  illustrated  in  Fig.  14,  Plate  III. 
It  is  not  a  sanidin,  however,  but  probably  a  labradorite  crystal. 

Groundmass. — The  grouudmass  of  the  more  trachytic  varieties  is  entirely 
crystalline,  though  never  granular  like  some  of  the  older  hornblende-ande- 
sites;  its  texture  is  also  very  loose  and  open,  a  fact  which  often  influences 
the  course  of  decomposition.  To  the  north  of  the  Utah  patches  of  glass 
similar  to  that  which  is  included  in  the  feldspars  of  the  same  locality  are 
distributed  through  the  groundmass,  and  on  the  ridge  running  east  by  south 
from  the  Geiger  Grade  toll-house,  D.  1,  as  well  as  at  the  point  whei'e  the 
Grade  cuts  the  younger  hornblende-andesite  area,  the  glass  prevails  to  such 
an  extent  that  the  rock  approaches  an  obsidian  in  character.  Its  pitchy 
black  color  is  due  merely  to  the  bisilicates  and  magnetite,  the  glass  and 
feldspar  being  transparent. 

Field  character. — Thc  more  trachytic  varieties  near  Shaft  III.  of  the  Siitro 
Tunnel,  and  on  the  southwesterly  spur  of  Mount  Rose,  are  red  or  purple, 
and  highly  porphyritic,  very  soft  and  rough  rocks,  quite  incapable  of  being 
confounded  with  any  other  occurrence  in  the  district.  They  do  not  exhibit 
regular  partings  or  columnar  structure.  Mount  Rose  and  Mount  Emma  are 
largely  composed  of  tufa  and  tufaceous  breccia.     The  tufa  is  not  macro- 


70  GEOLOGY  OF  THE  COMSTOCK  LODE. 

scopically  distinguishable  from  other  tufas,  such  as  that  of  augite-andesite, 
but  inclosed  masses  commonly  indicate  its  character.  The  exposure  repre- 
sented in  Plate  VII.  is  made  up  of  coarse  porphyries  and  tufa,  and  the  engrav- 
ing shows  a  species  of  bedding  in  the  rock,  no  doubt  due  to  variations  in 
eruption.  Gray,  tolerably  firm  varieties,  about  as  coarse  as  ordinary  gran- 
ular diorite,  occur  at  the  Sugar  Loaf,  F.  3,  and  near  the  Utah.  At  the  latter 
point  columnar  structure  is  very  finely  developed.  Mount  Abbie,  C.  2,  is 
intermediate  between  the  firm  gray  and  the  soft,  highly  porphyritic  modifica- 
tions, and  the  black  glassy  occurrences  require  no  further  description.  None 
of  these  bear  much  resemblance  to  the  prevalent  varieties  of  earlier  horn- 
blende-andesite,  but  there  is  a  considerable  area  to  the  northeast  of  Mount 
Emma,  and  just  outside  of  the  map,  where  the  resemblance  is  almost  perfect. 
This  area  seems  to  be  strictly  continuous  with  the  more  typical  one,  how- 
ever, and  transitions  occur.  Lithologically  the  presence  of  more  or  less 
mica  seems  characteristic. 

The  weathering  of  this  rock  is  commonly  confined  to  the  separation  of 
ferric  oxide,  not  merely  on  the  surface,  but  often  for  considerable  distance 
into  the  mass,  where  the  latter  is  of  an  open  texture.  In  the  neighborhood 
of  the  Sierra  Nevada  mine,  however^  chloritic  degeneration  of  the  bisili- 
cates  is  perceptible. 

Distinctive  characteristics. — No  csscutial  pfopcrty  dlstiuguishes  the  younger 
from  the  older  hornblende-andesite,  but  in  the  Washoe  District  it  forms  a 
variet}^  of  andesite  readily  distinguishable  in  most  cases  by  its  loose  struct- 
ure, and  the  presence  of  mica.  The  glassy  modification  is  more  likely  to  be 
confounded  with  augite-andesite,  but  the  luster  is  not  resinous,  as  in  the 
augitic  rocks.  The  distinction  is  hardest  to  draw  in  the  wild  canons  east 
of  Mount  Kate,  a  region  wholly  unlikely  ever  to  have  any  importance. 


BASALT. 

Basalt  plays  a  very  small  part  in  the  geology  of  the  district,  but  the 
rock  is  a  thoroughly  characteristic  representative  of  the  species.  It  is  dark 
and  compact,  with  many  visible  crystals  of  dark  amber-colored  olivine. 

Microscopical  character. — The  basalt  is  a  tlioroughly  crystalline  mixture  of 


THE  EOCKS  OF  THE  WASHOE  DISTRICT.  71 

olivine,  augite,  labradorite,  and  magnetite,  showing  no  glass  excepting  as 
inclusions  in  the  augites.  The  olivine  occurs  in  part  as  fairly  well  developed 
crystals,  with  hexagonal  and  octagonal  sections,  and  occasional  perceptible 
cleavages.  It  is  almost  colorless,  but  shows  the  faintest  possible  tinge  of 
yellow.  The  decomposition  amounts  only  to  a  slight  discoloration  along 
some  of  the  edges  and  cracks.  Augite  is  present,  in  part  in  crystals  as  large 
as  the  olivine,  and  in  part  in  minute  grains  forming  a  portion  of  the  ground- 
mass.  The  feldspar  is  crystallized  for  the  most  part  in  lath-like  forms,  and 
is  often  twinned  according  to  the  Carlsbad  law,  but  in  one  or  two  cases 
both  albitic  and  periclinic  twinning  are  visible.  The  determinable  crystals 
seem  all  to  belong  to  labradorite.     The  magnetite  is  in  no  way  remarkable. 

Field  character. — Tho  larger  part  of  the  basalt  occurs  in  the  form  of  ridges 
with  horizontal  summits,  giving  the  impression  of  tables,  though  they  are 
really  very  narrow.  At  the  base  of  these  ridges  are  numerous  bowlders 
which,  under  the  action  of  frost,  have  assumed  rounded  forms.  Besides  the 
areas  visible  on  the  map,  there  is  a  single  bluff-like  cropping  near  McClellan 
Peak,  where  the  bowlders  have  assumed  an  almost  perfectly  spherical  shape. 
It  is  hard,  and  rings  like  cast  iron  under  the  hammer,  but  is  rather  brittle 
and  chips  readily.  There  is  no  considerable  quantity  of  decomposed  basalt 
to  be  seen. 

This  rock  cannot  be  confounded  with  any  other  in  the  district,  for  it 
all  carries  visible  olivine,  a  mineral  not  met  with  in  any  other  Washoe  rock. 
The  elevation  laid  down  as  Basalt  Hill  is  augite-andesite,  and  the  rock 
described  by  Professor  Zirkel  as  an  unusual  basalt^  is  both  macroscopically 
and  microscopically  the  same  as  that  here  considered  as  metamorphic  diorite. 

'  Expl.  of  the  40th  Par.,  Vol.  VI.,  slide  528. 


72  GEOLOGY  OF  THE  COMSTOCK  LODE. 


Section2.    (Chapter  III.) 
THE  DECOMPOSITION  OF  THE  EOCKS. 

Sucli  facts  as  have  been  established  with  reference  to  the  decomposi- 
tion of  the  Washoe  rocks  are  necessarily  mentioned  in  connection  with  the 
lithological  description  of  each  species.  The  subject,  however,  is  one  of 
such  great  importance  in  the  geology  of  the  District  that  it  appears  advisa- 
ble to  consider  the  observations  bearing  upon  it  as  a  whole,  and  in  some 
detail. 

Area  of  extreme  decomposition. — While  fow  absolutcly  fresli  Tocks  occur  iu  tho 
region  surveyed,  decomposition  so  great  as  to  oppose  a  serious  obstacle  to 
lithological  determinations  is  confined  to  a  smaller  area.  In  the  nature  of 
things  this  area  is  incapable  of  precise  definition,  but  it  is  shown  as  nearly 
as  may  be  in  its  relation  to  the  Comstock  and  the  Occidental  lode  by  the 
accompanying  sketch  map,  page  73.  From  this  it  appears  that  precisely 
the  area  which  is  of  the  most  importance  in  a  discussion  of  the  vein-geology 
is  that  profoundly  decomposed. 

Effects  of  decomposition  on  various  rocks  the  same. Willie    tllC     pliysical     charaCtCr    of 

the  different  rocks  has  to  some  extent  modified  the  physical  results  of  decom- 
position, the  chemical  and  mineralogical  changes  and  the  degree  of  alter- 
ation observed  in  the  rocks  of  the  decomposed  area  seems  almost  wholly 
independent  of  their  age  or  species.  Granular  diorites,  porphyritic  diorites, 
the  two  diabases,  earlier  hornblende-andesite,  and  augite-andesite  appear  to 
have  been  subjected  to  the  same  influences,  with  the  same  results.  Quartz- 
porphyry  and  younger  hornblende-andesite  come  within  the  limits  of  the 
chief  area  of  decomjDosition  only  to  a  slight  extent,  either  above  or  below 
ground,  but  to  that  extent  they  show  the  same  effects,  as  does  also  the  met- 
amorphic-diorite  in  limited  spots  more  or  less  nearly  related  to  the  focus  of 
action.  Only  basalt  and  granite  have  escaped  with  mere  traces  of  decom- 
position, while  the  quartz-porphyry  as  a  whole  appears  to  have  been  sub- 


THE  DECOMPOSITION  OF  THE  ROCKS. 


73 


jected  to  decomposing  influences  not  shared  by  the  other  rocks  in  the  same 
degree.  It  is  difficult  to  avoid  the  conclusion  that  the  period  of  intense 
chemical  action  cannot  antedate  the  eruption  of  later  hornblende-andesite, 
and  probably  succeeded  it. 


Fig.  1. — Area  of  extreme  decomposition. 


Not  only  have  the  same  minerals  in  the  various  rocks  undergone  iden- 
tical processes  of  alteration,  but  similar  groups  of  minerals  have  yielded 
almost  identical  results.  Hoi-nblende,  augite,  and  mica  have  given  place  to 
the  same  ultimate  jiroducts,  though  in  slightly  different  proportions,  and  the 


74  GEOLOGY  OP  THE  COMSTOOK  LODE. 

degeneration  of  each  of  the  various  feldspar  species  has  taken  the  same 
course. 

Hornblende. — Homblcnde  in  the  diorites  is  met  with,  both  brown  and  green. 
The  brown  variety  is  usually  quite  fresh,  while  the  green  exhibits  a  tendency 
to  a  general  degeneration  throughout  its  whole  mass.  In  one  instance,  at 
least,  it  has  been  shown  that  the  brown  solid  hornblende  of  a  semi-porphy- 
ritic  diorite  is  altered  into  the  green  fibrous  modification,  and  in  other  cases 
there  is  strong  reason  to  suspect  a  like  change.  Similarly,  it  has  been  shown 
that  the  hornblende  of  the  metamorphic  diorite  was  in  all  probability  once 
colorless,  and  that  it  is  now  in  part  converted  into  a  green  modification  of 
a  fibrous  texture.  The  result  in  both  cases  is  very  similar  to  uralite.  It  is 
by  no  means  asserted  that  all  the  green  fibrous  hornblende  of  the  diorites 
in  Washoe  is  an  alteration-product  of  other  varieties,  though  this  seems 
possible,  but  there  is  evidence  enough  to  warrant  calling  the  attention  of 
lithologists  to  the  question  how  far  green  fibrous  hornblende  is  to  be  con- 
sidered the  original  form  of  the  mineral.  Professor  Rosenbusch  mentions 
this  change  •  in  connection  with  the  proterobases  of  Lusatia.  In  the 
younger  rocks  I  have  not  succeeded  in  detecting  a  similar  change.  The 
hornblendes  of  the  Washoe  andesites  are  either  full  brown,  reddish  brown, 
or  greenish  brown  in  color.  The  tint  of  those  last  mentioned  it  is  somewhat 
difficult  to  describe,  and  consultation  has  shown  that  the  definition  proposed 
depends  considerably  on  the  susceptibility  of  different  eyes.  To  some  they 
appear  green  with  a  tinge  of  brown,  while  to  others  the  green  admixture 
is  scarcely  perceptible;  but  all  agree  that  the  color  is  very  different  from 
the  grass-green  or  bluish-green  of  the  fibrous  diorite  hornblendes. 

Alteration  of  hornblende  to  chlorite. — Thc  fibrous  dloHtic  homblcnde,  somc  of  the 
brown  variety  in  the  por^jhyritic  diorites,  and  all  the  hornblendes  of  the 
younger  rocks,  appear  to  pass  directly  into  chlorite.  The  attack  seems  to 
take  place  from  external  surfaces  and  cracks.  If  the  cleavages  of  the  crystal 
are  well  opened,  each  cleavage  prism  is  attacked,  and  the  result  in  longitu- 
dinal section  is  a  quasi-fibrated  mass  of  rods  of  hornblende  separated  by 
chlorite,  and  in  cross-section  a  group  of  isolated  rhombs  or  irregular  patches 
of  the  unaltered  mineral  embedded  in  chlorite,  which  often  retains  the  outlines 
of  the  oi'iginal  crystal  in  great  perfection.     Figs.  1,  2,  and  3,  Plate  II.,  are 


THE  DECOMPOSITION  OP  THE  KOOKS.  75 

exact  representations  of  such  cases.  The  chlorite,  with  only  one  or  two  ex- 
ceptions, has  the  same  characters  described  elsewhere.^  The  Washoe  chlorite 
evinces  a  considerable  solubility,  which  can  be  traced  in  many  series  of  slides, 
notably  in  those  from  the  McKibben  Tunnel.  In  the  early  stages  the  pseudo- 
morphs  after  hornblende  are  verj^  fine;  later  the  form  of  the  hornblendes  is 
obscured,  and  irregular  patches  of  chlorite  appear  in  the  groundmass;  at 
last  this  mineral  appears  diffused  through  the  rock,  settling  in  bands  round 
apatites,  magnetites,  or  other  solid  minerals,  and  penetrating  partially  decom- 
posed feldspars.  Frequently  too  it  occupies  microscopic  veins  traversing  the 
slide.  One  such  observation  would  perhaps  be  open  to  great  question,  but 
scores  of  similar  cases  occur  in  the  numerous  slides  examined. 

Augite  and  mica. — Both  augitc  aud  mica  exhibit  the  same  tendency  to  pass 
into  chlorite  as  hornblende,  and  neither  the  process  nor  the  result  commonly 
differs  in  any  way  from  that  just  described.  A  preliminary  change  of  augite 
touralite  is,  however,  not  uncommon  in  the  diabases,  and  in  a  single  slide  of 
augite-andesite  the  same  alteration  appears,  though  several  other  thin  sections 
from  the  same  cropping  show  nothing  of  it.  On  the  whole  augite  seems  to 
be  somewhat  more  disposed  to  decomposition  than  hornblende,  and  cases  are 
numerous  in  which  the  hornblendes  of  a  rock  retain  their  freshness,  when 
the  augites  are  completely  altered;  but  in  some  instances  the  augites 
have  resisted  longer  than  the  hornblendes.  Mica,  on  the  other  hand,  cer- 
tainly yields  somewhat  less  readily  than  the  bisilicates,  to  which  it  is  so 
closely  allied,  though  it  is  often  wholly  changed  to  chlorite. 

Formation  of  pyrite. — lu  Very  numcrous  cascs  pyrite  has  been  observed  in  rela- 
tions indicating  that  it  is  formed  directly  from  hornblende  and  augite,  appar- 
ently at  the  same  time  as  chlorite.  The  two  products  do  not  seem  to  me 
dependent  upon  one  another,  for  chlorite  occurs  where  no  pyrite  is  found, 
and  the  process  of  conversion  to  chlorite  is  not  visibly  modified  by  the  simul- 
taneous growth  of  pyrite.  The  indications  are,  therefore,  that  the  two  pro- 
cesses are  wholly  independent  of  and  not  inconsistent  with  each  other. 

Epidote  formed  from  chlorite. — Epidotc  is  usually  cousidcred  as  a  direct  result 
of  the  decomposition  of  the  bisilicates,  but  in  Washoe  such  a  transforma- 
tion, if  it  occurs,  must  be  exceptional,  for  it  was  not  recognized  in  a  single 

1  See  pp.  84,  211,  etc. 


76  GEOLOGY  OF  THE  COMSTOCK  LODE. 

instance,  though  the  paragenesis  of  the  products  of  decomposition  was  a 
subject  of  special  inquiry.  On  the  other  hand,  the  formation  of  epidote  at 
the  expense  of  chlorite  is  proved  beyond  a  doubt.  The  development  of 
epidote  usually  begins  near  the  centers  of  patches  of  chlorite,  sending  out 
faggot-like  masses  of  crystals  in  all  directions,  and  ultimately,  under  cer- 
tain conditions,  occupying  the  whole  space.  In  certain  stages  this  process 
can  be  admirably  observed,  long  prismatic  needles  of  epidote  extending  into 
the  chlorite  and  cutting  the  minute  iibers  of  the  latter  at  all  sorts  of  angles. 
This  is  peculiarly  well  seen  in  Figs.  6  and  7,  Plate  II.  Sometimes  the 
chlorite-fibers,  at  the  periphery  of  hornblende  pseudomorphs,  exhibit  a  spec- 
ial arrangement,  lying  strictly  parallel  to  one  another  and  perpendicularly 
to  the  crystal  face.  These  fibers  are  often  nearly  of  the  same  length,  and 
thus  form  a  belt  or  zone.  Such  a  belt  must  be  denser  than  a  spherolitic 
mass,  and  not  infrequently  appears  to  offer  a  greater  resistance  to  the  forma- 
tion of  epidote.  This  is  in  accordance  with  the  chemical  conditions,  for  the 
transformation  cannot  take  place  without  the  access  of  solutions.  Complete 
pseudomorphs  of  epidote  after  hornblende  may  result  from  this  process  under 
favorable  conditions,  and  such  an  one  occurring  in  a  slide  from  the  McKihhen 
Tunnel  is  shown  in  Fig.  9,  Plate  II.  From  a  study  of  a  series  of  slides  from 
tlie  same  locality  it  is  evident  that  this  pseudomorph  is  the  last  stage  of  the 
process  illustrated  by  Figs.  6  and  7,  Plate  II.,  and  not  a  case  of  direct  con- 
version. Epidote  is  also  constantly  found  developing  in  patches  of  chlorite, 
which  occur  in  the  groundmass  of  the  rocks,  where  they  have  apparently 
been  deposited  but  not  formed;  and  microscopic  veins  of  chlorite  are 
common  in  which  various  proportions  of  the  mass  are  changed  to  epidote. 
Feldspar  docs  not  decompose  to  epidote. — When  the  feldspars  bccomo  porous,  as 
they  do  so  soon  as  decomposition  has  commenced,  they  are  subject  to  infil- 
tration by  chlorite,  and  the  chlorite  so  deposited  is  converted  into  epidote 
under  the  same  conditions  as  in  other  portions  of  the  rock.  One  distin- 
guished lithologist  has  attributed  the  supposed,  but  confessedly  mysterious, 
alteration  of  feldspar  into  epidote,  to  the  presence  of  plentiful  hornblende - 
needles  embedded  in  the  feldspar.  In  the  section  of  this  chapter  dealing 
with  propylite  it  is  shown  that  this  determination  is  erroneous,  the  supposed 
hornblende  particles  in  the  sHde  upon  which  the  suggestion  is  founded  being 


THE  DECOMPOSITION  OF  THE  ROCKS.  77 

in  fact  chlorite;  but  that  the  epidote  is  to  be  attributed  to  the  alteration  of 
these  foreign  jjarticles,  and  not  to  a  transformation  of  the  feldspar  sub- 
stance, seems  to  me  certain.  The  grounds  for  this  view,  which  has  not 
hitherto  been  entertained,  are  as  follows:  There  is  no  question  that  chlorite 
arises  from  the  decomposition  of  the  bisilicates,  and  that  it  may  become 
diffused  through  the  groundmass  is  equally  certain.  Many  slides  from 
Washoe  show  the  feldspars  in  a  very  fresh  state,  while  the  bisilicates  are 
wholly  chloritized.  In  such  cases  the  chlorite  cannot  be  due  to  feldspar 
decomposition,  but  its  diffusion  through  the  groundmass  is  nevertheless 
common.  Carious  feldspars  appear  to  be  impregnated  with  chlorite  as  a 
rule  when  the  neighboring  bisilicates  are  undergoing  chloritic  decompo- 
sition, but  not  otherwise;  and  epidote  is  found  developing  in  chloritic 
masses  inclosed  in  feldspar  when,  and  only  when,  the  same  process  is 
going  on  in  chlorite  patches  not  so  inclosed,  the  origin  of  which  is  distinctly 
referable  to  hornblende,  augite,  or  mica.  Many  cases  of  the  formation  of 
epidote  have  been  observed  in  chlorite  inclosed  in  feldspar,  as  convincing 
as  the  instances  of  the  transformation  of  chlorite  arising  from  the  bisilicates 
which  are  illustrated  on  Plate  II.,  though  none  so  beautiful;  but  in  no 
instance  in  the  Washoe  District  has  epidote  been  seen  sending  its  twig-like 
crystals  into  feldsjDathic  masses. 

Other  alteration-products  of  chlorite. — Chlorlte  also  dcgcuerates  iuto  quartz,  cal- 
cite,  and  limonite,  and  this  change  is  sometimes  to  be  observed  in  the  same 
slide  which  shows  its  alteration  to  epidote.  In  this  case,  also,  the  dense  belt 
of  chlorite  which  occasionally  forms  at  the  surface  of  a  crystal  of  hornblende, 
seems  to  offer  considerable  resistance  to  attack.  An  instance  is  illustrated 
in  Fig.  10,  Plate  II.  Sometimes  this  change  seems  to  be  referable  to  a  dis- 
tinct period  in  the  decomposition  of  the  rock,  as  in  the  case  shown  in  Fig. 
3,  Plate  II.,  and  described  under  slide  464. 

Character  of  the  chloritic  mineral. — The  chloritc  arisiug  from  the  bisilicatcs  and 
mica  shows  the  same  optical  properties,  and  the  conversion  of  chlorite  into 
epidote  is  frequent  from  whichever  primary  mineral  it  may  be  derived. 
There  seems,  however,  a  somewhat  smaller  tendency  for  the  chlorite  arising 
from  augite  to  change  to  epidote,  than  is  displayed  by  that  formed  from 
hornblende  and  mica;  but  the  difference  in  this  respect  is  not  great  or  uniform. 


78  GEOLOGY  OF  THE  GOMSTOCK  LODE. 

Insolubility  of  epidote. — Epidote  is  usuallj  classed  as  an  insoluble  mineral,  and 
the  evidence  to  the  contrary  in  the  Washoe  District  is  slight.  Epidote,  it 
is  true,  frequently  crystallizes  in  vugs,  but  since  chlorite  certainly  possesses 
some  degree  of  solubility,  their  growth  might  be  accounted  for  by  supposing 
them  to  form  in  a  solution  of  the  mineral  from  which  they  are  derived. 
There  seems,  however,  to  be  a  relation  between  the  size  of  epidote  masses 
and  of  the  crystalline  grains  of  which  they  are  composed,  which  is  most 
easily  accounted  for  by  supposing  the  mineral  to  be  somewhat  soluble.  In 
very  small  patches  epidote  is  frequently  so  fine-grained  as  to  reflect  almost 
all  the  light,  and  under  low  powers  appears  opaque.  In  larger  masses, 
formed  apparently  under  similar  conditions,  the  epidote  often  shows  crystal- 
line grains  of  considerable  size,  and  transmits  light  readily.  In  a  few  cases 
among  the  diabases  epidote  appears  to  be  replaced  by  opaque  mixtures  of 
iron  oxide  and  other  substances,  but  no  certain  instance  of  this  sort  was 
made  out,  and  there  is  usually  no  indication  of  any  tendency  to  decompose. 

Decomposition  of  feldspars. — The  study  of  thc  proccss  of  decomposition  which 
the  feldspars  of  the  Washoe  rocks  undergo  is  much  less  satisfactory.  They 
have  offered  a  far  greater  resistance  than  the  bisilicates,  and  no  great  con- 
tinuous area  exists  in  which  they  are  not  sufficiently  fresh  to  be  readily 
determinable.  Incipient  decomposition  is  marked  by  the  appearance  of 
specks  of  calcite,  readily  recognizable  in  polarized  light.  At  a  later  stage 
quartz  grains  make  their  appearance,  accompanied  by  particles  of  a  white 
opaque  substance,  of  a  nature  unknown  to  me.  In  the  last  stages  of  decom- 
position nothing  further  than  these  three  substances  is  recognizable.  Kaolin, 
according  to  Mr.  H.  Fischer,  is  an  isotropic  substance,  accompanied  in  the 
slides  he  studied  by  polarizing  grains  and  scales.^  Nacrite  is  crystalline  and 
consists  of  an  aggregate  of  six-sided  scales  of  fibrous  texture,  each  composed 
of  six  triangular  sectors.  Nothing  corresponding  to  the  description  of 
either  was  observed  in  any  of  the  slides,  a  fact  which  seems  to  prove  that 
kaolinization,  if  it  has  taken  place  at  all,  is  a  very  subordinate  phenomenon. 
The  analyses  of  the  ''clays,"  too,  show  that  they  are  not  concentrations  of 
kaolin  washed  out  of  the  surrounding  rocks,  but  represent  so  much  rock 
crushed  and  degenerated  in  place.     The  water  contents  of  some  of  them  is 

'Eosenbusch:  Phys.  derMin.  u.  Gesteine,  Vol.  I.,  p.  .374. 


THE  DECOMPOSITION  OF  THE  EOCKS.  79 

such  as  to  preclude  the  idea  that  even  this  material  contains  any  notable 
quantity  of  kaolin. 

Secondary  liquid  inclusions. — The  behavior  of  the  parti cles  of  calcite  which  first 
form  in  the  feldspars  is  of  some  little  importance,  for  these  on  leaching  out 
give  rise  to  secondary  liquid  inclusions,  as  will  be  explained  under  slide  210.' 
Such  inclusions  are  met  in  all  the  decomposed  andesites  and  appear  to  be 
frequent  also  among  the  other  rocks.  If  proper  regard  is  paid  to  the  con- 
dition of  the  feldspars  and  to  the  shape  of  the  inclusion,  there  is  little 
difficulty  in  discriminating  between  secondary  and  primitive  liquid  inclu- 
sions, but  a  neglect  of  these  precautions  might  readily  lead  to  incoiTect 
diagnoses. 

Magnetite  appears  in  certain  cases  to  be  converted  into  a  yellowish- 
white  opaque  substance,  accompanied  by  polarizing  grains,  much  resem- 
bling calcite.  The  black  border  of  hornblendes  is  sometimes  wholly 
removed  in  this  manner,  and  the  appearance  of  the  rock  considerably 
modified.  The  phenomena  suggest  a  conversion  to  a  mixture  of  carbonate 
of  iron  and  limonite. 

Decomposition  of  rock-masses. — The  course  taken  by  the  decomposition  of  masses 
of  rock  depends  largely  on  their  physical  character,  and  is  sufficiently  dis- 
cussed in  connection  with  the  general  description  of  each  rock.  Only  porous 
masses  suffer  decomposition  uniformly  throughout,  and  these  are  apt  to  retain 
their  coherence.  Blocks  of  dense  rocks  are  attacked  from  their  surfaces  and, 
as  in  all  processes  involving  solution  or  substitution,  the  corners  and  edges 
yield  more  rapidly  than  the  flat  faces,  so  that  the  fresh  kernel  tends  to  assume 
a  spheroidal  shape.  The  altered  portion  of  the  dense  rocks  frequently  dis- 
integrates. 

Condition  of  the  quartz-porphyry. — The  quartz-porphyry  throughout  the  District 
and  far  beyond  its  limits  is  so  much  decomposed  that  not  a  single  fresh  horn- 
blende has  anywhere  been  found  in  it.  Except  where  increased  by  special 
causes,  such  as  propinquity  to  the  Lode,  the  degree  of  alteration  is  also  very 
uniform.  As  it  overlies  very  fresh  granite  and  metamorphic  diorite  and  is 
overlain  by  fresh  andesites,  special  causes  must  be  sought  to  account  for  its 
exceptional  degeneration.     None  such  have  occurred  to  me  except  its  phys- 

'Page  119. 


80  GEOLOGY  OF  THE  COMSTOOK  LODE.  - 

ical  structure.  The  porphyry  is,  and  seems  always  to  have  been,  very 
porous,  and  has  permitt'ed  a  more  rapid  percolation  of  surface  waters  than 
the  other  rocks.  This  is  not  improbably  ascribable  to  the  difference  in  the 
coefficient  of  expansion  of  the  quartz  grains,  and  the  other  mineralogical 
constituents. 

The  chemical  aspects  of  the  decomposition  of  the  Washoe  rocks  will 
be  discussed  in  a  separate  chapter. 


PROPTLITE.  81 


Section   3.     (Chapter  III.) 

PROPTLITE. 

Historical  statement. — The  term  propyHtc,  as  is  well  known,  was  introduced 
into  lithology  by  Baron  F.  v.  Richthofen,  mainly  in  consequence  of  observa- 
tions made  in  the  Carpathians  and  in  the  States  of  California  and  Nevada.  In 
his  memoir  on  "The  Natural  System  of  Volcanic  Rocks  "^  greater  prominence 
is  given  to  the  Washoe  occurrence  than  to  any  other.  From  his  description 
of  the  rock  the  following  statement  of  its  characteristics  is  taken  almost 
verbatim.  Propylite  is  always  porphyritic,  and  no  prominent  property  dis- 
tinguishes it  from  porphyritic  diorite.  The  feldspars  are  oligoclase  and  the 
hornblendes  ordinarily  dark-green  and  fibrous.  The  groundmass  is  usually 
green  and  appears  to  owe  its  color  to  the  profuse  dissemination  of  small 
particles  of  fibrous  hornblende.  It  also  presents  a  peculiar  and  recog- 
nizable, though  hardly  describable,  appearance-  or  habitus.  It  is  extra- 
ordinarily rich  in  mineral  veins,  both  in  Europe  and  in  America.  Geo- 
logically it  is  the  earliest  of  the  Tertiary  volcanic  rocks.  Mr.  Clarence 
King^  accepted  Baron  v.  Richthofen's  determination  of  propylite  in  the 
Washoe  District  (a  region  which  he  visited  in  company  with  that  geologist), 
though  with  some  limitations  and  additions.  Outside  of  this  District  the 
geologists  of  the  Exploration  of  the  Fortieth  Parallel  found  only  a  few 
obscure  localities  of  the  rock.  In  1876  Prof  F.  ZirkeP  confirmed  the 
independence  of  proi^ylite  as  the  result  of  a  microscopical  examination  of 
the  collections  of  the  Exploration  of  the  Fortieth  Parallel.  In  1880  Capt. 
C.  E.  Dutton''  announced  the  presence  of  considerable  areas  of  propylite 
in  Utah. 

'Mem.  Cal.  Acad,  of  Sciences,  Vol.  I.,  Part  II.       ^Exploration  of  the  Fortieth  Parallel,  Vol.  III. 
3  Ibid.,  Vol.  VI.  ^Tho  High  Plateaus  of  Utah. 

6   C   L 


82  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Failure  of  the  search  for  propyiite. — Washoe  presenting  tlio  typical  Ameiican  oc- 
currence of  propyiite,  a  study  of  the  rock  necessarily  formed  a  prominent 
feature  in  the  re-examination  of  the  District;  for  while  the  structure  and  vein 
formation  of  the  Comstock  are  the  objects  of  first  importance  and  interest  in 
Washoe,  the  first  step  toward  their  elucidation  was  manifestly  to  clear  up  the 
lithological  obscurities  as  far  as  possible.  Since  Baron  v.  Richthofen  and  Mr. 
King  examined  the  District,  the  exposures  of  rock  have  been  greatly  increased. 
Not  only  have  the  mines  on  the  Lode  been  deepened  by  a  couple  of  thousand 
feet,  but  innumerable  roads,  quarries,  prospect-holes,  and  the  like,  have  ex- 
posed more  than  the  mere  weathered  surface  in  thousands  of  spots.  It  soon 
became  apparent  that  the  area  of  andesite,  which  to  Baron  v.  Richthofen 
seemed  inconsiderable  and  to  Mr.  King  quite  subordinate  to  that  of  the  . 
propyiite,  had  been  unden-ated.  Fresh  andesites  were  found  exposed  by 
cuts  in  many  localities  which  had  been  laid  down  as  propyiite;  and  since 
the  latter  was  supposed  to  underlie  the  former,  the  upper  portions  of  these 
exposures  furnished  a  safe  study  of  decomposed  andesites,  the  results  of 
which  could  be  applied  elsewhere.  It  was  found  that  even  where  a  high 
degree  of  decomposition  and  a  thoroughly  propylitic  character  prevailed, 
reasonably  fresh  rocks  could  be  discovered  by  diligent  search,  either  as 
masses  protected  by  some  accidental  arrangement  of  fissures,  or  as  nodules 
at  the  centers  of  concentrically  weathered  blocks;  and  to  the  east  of  the 
Lode,  wherever  fresh  rocks  were  discovered  among  the  propylites,  they 
always  proved  andesitic.  Where  andesite  dikes  or  overflows  had  been 
recognized,  and  had  been  supposed  to  succeed  propyiite,  careful  examina- 
tion and  excavation  showed  that  the  change  was  through  a  transition,  not 
by  a  contact.  In  short,  the  propyiite  area  to  the  east  of  the  Lode  was 
reduced  almost  foot  by  foot,  until  it  disappeared  altogether.  The  propyhte 
from  the  head  of  Ophir  ravine,  one  of  the  type-localities,  had  a  slightly 
diff"erent  character  from  the  eastern  rock,  yet  the  diff"erence  was  not  greater 
than  seemed  possible  within  the  limits  of  a  rock-species.  Fortunately 
there  are  many  long  tunnels  penetrating  the  hills  in  the  neighborhood;  and 
an  examination,  undertaken  to  establish  contacts  between  propyiite  and 
diorite,  resulted  in  a  study  of  transitions  between  typical  diorite-porphyries 
and  decomposed  porphyritic  forms  of  the  same  rock.     At  last  even  in  the 


PKOPYLITE.  83 

huge  "propylite"  croppings  of  Ophir  Ravine  the  industrious  use  of  the  sledge 
revealed  surfaces  which  were  unmistakably  dioritic,  and  so  propylite  dis- 
appeared from  the  surface.  Under  ground  it  early  became  evident  that  the 
east  countiy  rock  was  different  from  that  upon  the  surface ;  but  a  long  time 
elapsed  before  an  accidentally  protected  mass  was  discovered,  which  was 
fresh  enough  to  serve  as  a  basis  for  determination.  It  proved  to  be  diabase. 
Later,  other  localities  of  fresh  diabase  were  found,  but  while  in  a  new  dis- 
trict broad  inferences  might  soon  have  been  drawn  as  to  the  character  of 
the  hanging  wall,  this  was  impossible  in  the  face  of  previous  determinations. 
If  a  rock  answering  to  the  definition  of  propylite  existed,  it  was  necessary 
to  determine  its  precise  area  and  occuiTcnce;  and  if  there  were  no  such 
rock  it  was  indispensable  to  prove  that  the  whole  area  was  occupied  by 
others.  The  state  of  decomposition  of  the  underground  rocks  is  so 
advanced,  that  in  not  more  than  three  out  of  a  hundred  of  the  specimens, 
all  selected  with  the  utmost  care,  is  there  a  fresh  augite  or  hornblende,  and 
perhaps  half  of  the  185  miles  of  underground  workings  are  accessible. 
The  task  was  therefore  a  laborious  one.  The  lithological  examination 
became  a  protracted  study  of  decomposition-products,  and  resulted  in 
proving  that  propylite  did  not  exist  below  the  surface  any  more  than 
upon  it. 

propyiitic  habitus. — The  uiost  Striking  macroscopical  points  of  distinction 
between  the  rocks  in  the  Washoe  District  which  have  been  determined  as 
propylite,^  and  the  better  established  Tertiary  and  ante-Tertiary  rocks,  are 
a  greenish  color  which  often  tinges  the  feldspars  as  well  as  the  groundmass, 
impellucid  feldspars,  and  a  certain  blending  of  the  mineral  ingredients 
which  helps  to  deprive  the  rock  of  those  characteristics  by  which  we  are 
accustomed  to  recognize  fresh  specimens  as  belonging  to  the  older  or  to  the 
younger  series.  These  appearances  seem  to  me  to  constitute  its  "charac- 
teristic habitus." 

Fallacious  nature  of  the  distinguishing  characteristics. BarOU  VOn    Richthofeil  bclicVed 

that  the  macroscopical  character  of  the  rock  was  due  to  green,  fibrous 
hornblende,  and  the  diffusion  of  this  mineral  in  fibrous  particles  through 
the  mass  of  the  rock.     This  view  was  confirmed  by  Professor  Zirkel,  who 

'  See  page  8cl. 


84  GEOLOGY  OF  TBE  COMSTOCK  LODE. 

founds  the  greater  part  of  his  diagnostic  points  of  difference  between  pro- 
pylite  and  andesite  upon  the  color  and  structure  of  the  hornblende,  and  its 
distribution  in  the  rock.  What  has  been  taken  for  green  fibrous  hornblende, 
however,  in  a  great  majority  of  the  propylite  slides  of  the  collection  of  the 
Exploration  of  the  Fortieth  Parallel  proves  to.be  not  hornblende,  but  chlo- 
rite. This  mineral,  which  is  probably  the  rhipidolite  of  G.  Rose,  is,  like  horn- 
blende, green,  fibrous,  and  strongly  dichroitic,  but  it  occurs  largely  in 
spherolitic  and  felt-like  masses,  extinguishes  light  when  either  of  the 
principal  sections  of  the  polarizing  apparatus  is  parallel  to  the  fibers; 
and,  when  the  Nicols  are  crossed,  usually  shows  only  dark-bluish  tints, 
very  different  from  those  commonly  transmitted  by  hornblende.  In  one  of 
the  slides,  indeed,  there  is  abundant  green  fibrous  hornblende,  but  the  rock 
is  a  granular  diorite  from  Mount  Davidson,  while  in  the  section  from  Storm 
Canon,  Fish  Creek  Mountains,  there  is  both  chlorite  and  hornblende,  but 
the  latter  is  certainly  uralite. 

It  has  been  shown  in  the  preceding  section  of  this  chapter  that  chlorite, 
which  is  a  decomposition-product  of  hornblende,  augite,  or  mica,  is  fre- 
quently diffused  through  the  groundmass  and  any  feldspars  which  may 
have  become  porous  through  decomposition.  This  fact,  combined  with  the 
mistake  of  chlorite  for  hornblende,  explains  the  distinctions  based  upon  the 
greenish  hue  of  the  propylitic  rocks,  upon  the  color  and  structure  of  the 
masses  mistaken  for  hornblende,  and  upon  the  distribution  of  supposed  par- 
ticles of  that  mineral  through  groundmass  and  feldspars.  Seemingly  con- 
clusive proof  has  also  been  offered  elsewhere  that  epidote  in  the  Washoe  Dis- 
trict is  not  an  immediate  product  of  the  decomposition  of  hornblende,  but 
of  chlorite ;  which  explains  its  absence  in  the  comparatively  fresh  rocks  recog- 
nized as  andesitic. 

In  one  limited  area  of  hornblende-andesite,  not  represented  among  the 
slides  of  the  Fortieth  Parallel,  minute  spiculse  of  hornblende  occur,  distrib- 
uted through  the  groundmass ;  but  they  are  brown,  each  microlite  is  solid, 
and  they  are  not  grouped  in  crystal-like  aggregates.  In  almost  all  cases 
the  andesitic  hornblendes  when  fresh  are  black-bordered;  but  while  the 
magnetite  usually  resists  decomposition  longer  than  the  hornblende  sub- 
stance, it  sometimes  yields  first.     The  hornblendes  of  the  diorite-porphyries, 


PEOPYLITB.  85 

though  otherwise  very  similar  to  those  of  the  andesites,  seldom  show  even 
a  trace  of  the  black  border.  Barring  two  or  three  exceedingly  local  excep- 
tions, a  division  of  the  hornblende  rocks  of  the  District  into  those  showing 
black-bordered  crystals,  and  those  which  do  not  contain  them,  would  be 
equivalent  to  a  separation  into  andesites  and  diorites.  The  assertion  that 
propylite  is  characterized  by  the  presence  of  hornblendes  without  black 
borders  is  founded  on  the  determination  of  chlointe  as  hornblende. 

Much  of  the  chlorite  mistaken  for  hornblende  is  due  to  the  decomposi- 
tion of  augite,  but  though  fine  pseudomorphs  of  this  description  occur  in 
the  slides  of  the  Fortieth  Parallel  collection,  the  significance  of  their  out- 
lines appears  to  have  been  ovei-looked.  Some  of  these  slides  are  from  typ- 
ical, though  somewhat  altered,  augite-andesites.  Augite  also  occurs  in  the 
dioritic  porphyries  of  Washoe. 

Glass  inclusions,  in  some  cases  partially  devitrified,  seem  to  occur  in 
nearly  all  the  propylitic  rocks  of  volcanic  origin,  while  they  are  of  course 
absent  from  the  dioritic  rocks  included  among  the  propylites.  The  Washoe 
andesites  are  somewhat  unusually  crystalline,  and  if  those  which  have  been 
regarded  as  propylite  ever  contained  any  isotropic  base,  of  which  there  is 
no  evidence  from  analogy,  it  is  now  devitrified. 

Quartz  occurs  to  a  considerable  extent  among  the  granular  diorites,  as 
an  original  constituent.  One  specimen  of  hornblende-andesite,  from  an 
area  not  represented  in  the  collection  of  the  Exploration  of  the  Fortieth 
Parallel,  however,  contains  a  few  minute  quartz  grains  of  indubitably  prim- 
itive character,  and  these  carry  fluid  inclusions.  Occasional  fluid  incliisions 
have  of  late  years  been  found  in  all  volcanic  rocks,  and  they  do  not  conse- 
quently form  a  conclusive  point  of  difference  unless  they  are  widely  dis- 
tributed and  are  present  in  great  abundance.  In  some  of  the  rocks  deter- 
mined by  Professor  Zirkel  as  quartz-propylite,  the  quartz  appears  to  me  to 
be  secondary.  It  occurs  in  groups  of  grains  of  different  orientation,  and  is 
indistinctly  separated  from  the  surrounding  mass.  Secondary  quartz,  of 
course,  frequently  contains  liquid  inclusions. 

Value  of  habitus  in  rock-determinations. — The  methods  cmploycd  to  identify  the 
propylites  of  Washoe  with  other  rocks  were  by  no  means  confined  to  mere 
mineralogical  examinations  under  the  microscope.    It  is  but  a  few  years  since 


86  GEOLOGY  OF  THE  COMSTOCK  LODE. 

the  only  resources  of  the  Hthologist  were  a  study  of  variations  and  transi- 
tions, and  a  keen  perception  of  the  habitus  characteristic  of  rock-species, 
aided  only  by  the  feeble  help  of  a  lens  and  an  occasional  chemical  analysis; 
and  how  much  can  be  accomplished  in  this  way  is  evident  from  the  fact  that 
the  chief  features  of  lithological  classification  are  still  much  what  they  were 
before  the  introduction  of  the  microscope.  Nor  are  the  methods  of  the 
older  lithologists  antiquated;  on  the  contrary,  the  proper  use  of  the  micro- 
scope greatly  increases  their  applicability  and  efficiency.  The  microscope 
enables  lithologists  of  the  present  day  to  give  greater  precision  to  their  ideas 
of  macroscopical  habitus,  and  to  distinguish  in  most  cases  between  essential 
and  non-essential  characteristics,  and,  with  this  advantage,  they  should 
become  even  keener  field  observers  than  their  predecessors.  Indeed  the 
relations  of  lithological  varieties,  and  of  the  causes  on  which  they  are  depend- 
ent, can  be  successfully  studied  only  in  the  field.  In  the  present  investiga- 
tion slides  were  ground  and  examined  from  day  to  day  as  the  exigencies  of 
the  field-work  seemed  to  demand.  The  microscopical  and  macroscopical 
appearances  were  also  diligently  compared  (for  grinding  slides  without  ma- 
chinery was  a  serious  addition  to  the  labor  of  days  spent  in  the  saddle  or 
under  ground) ;  and  it  became  possible  at  length  to  recognize  at  a  glance  a 
unity  of  origin  in  specimens  of  very  diverse  appearance  and  to  detect  litho- 
logical diff'erences  in  spite  of  advanced  decomposition  and  great  appai-ent 
similarity.  It  proved  possible  to  make  the  proper  allowance  for  decom^w- 
sition  and  to  infer  the  original  habitus  when  veiled  by  another  of  secondary 
origin,  as  well  as  to  identify  the  precise  character  of  the  change. 

Typical  propyiite  localities. — The  threc  most  important  propylite  localities  men- 
tioned by  Professor  Zirkel  in  the  Washoe  region  are  the  head  of  Ophir 
Ravine,  Crown  Point  Ravine,  and  Gold  Hill  Peak.  The  last  is  represented 
in  the  map  accompanying  the  present  report,  as  the  southern  Twin  Peak 
(C.4). 

Head  of  Ophir  Ravine. — The  uppcr  portiou  of  Ophir  Ravine  presents  a  very 
great  variety  of  diorite-porphyries,  which  are  not  related  as  separate 
flows  or  sheets,  but  pass  over  into  one  another  as  if  the  whole  heterogeneous 
mass  had  cooled  at  once.  The  character  of  the  rock  changes  every  few 
feet,  and  the  same  varieties  recur  in  spots.     Among  them  are  some  so  gran- 


PEOPYLITE.  87 

ular  as  to  be  nearly  indistinguishable  from  Mount  Davidson  rock,  while 
others  are  dark  fine-grained  porphyries  closely  resembling  andesites.  The 
latter,  however,  can  be  shown  from  slides  to  be  dioritic,  while  the  granular 
varieties  are  as  like  the  Mount  Davidson  rock  microscopically  as  they 
appear  to  the  naked  eye.  A  portion  of  these  rocks  is  altered,  but  the  transi- 
tions from  the  fresh  to  the  decomposed  state  can  be  studied  more  satisfactorily 
in  the  McKihhen  Tunnel,  because,  in  the  ravine,  decomposition  is  most  preva- 
lent in  the  bluffs  near  the  andesite,  which  is  also  somewhat  altered.  There 
is  no  evidence  of  any  contact  between  these  bluffs  and  the  unquestionable 
dioritic  masses  adjoining  them,  and  in  spots  where  the  rock  is  comparatively 
fresh,  its  character  seems  unmistakably  the  same ;  but  when  the  effect  of 
decomposition  on  the  tunnel  porphyries  is  considered  in  reference  to  the 
ravine  rocks,  it  becomes  clear  thdt  the  bluffs  can  be  only  altered  forms  of 
the  adjacent  varieties  of  diorite. 

Crown  Point  Ravine. — Ouc  flauk  of  Crowu  Poiut  Raviue  shows  tolerably  fresh 
hornblende-andesites,  the  other  excellent  fresh  augite-andesite.  Near  the 
drainage  the  rock  is  largely  a  highly  decomposed  breccia,  in  part  bleached 
to  whiteness,  but  the  area  occupied  by  propylitic  rocks  is  very  small  and 
could  only  represent  an  exposure  by  erosion.  As  is  common  in  breccias, 
the  decomposition  is  not  uniform.  The  matrix  is  so  altered  that  its  coher- 
ence is  a  matter  of  surprise,  and  many  of  the  included  fragments  are  tinged 
with  epidote.  Some,  however,  from  superior  density  or  accidental  protec- 
tion are  less  affected,  and  a  few  large  unfissured  blocks  are  tolerably  fresh 
at  some  distance  within  their  surfaces.  Wherever  the  inclosed  masses  are 
fairly  fresh  they  look  like  andesites,  and  under  the  microscope  there  proves 
to  be  no  distinction,  when  the  course  of  chlaritic  decomposition  known 
from  other  occurrences  is  allowed  for. 

South  Twin  Peak  ("Gold  Hill  Peak"). — The  South  Twlu  Peak  looks  more  like  the 
younger  gray  hornblende-andesite  of  the  Utah  quarry  than  the  more  usual 
varieties  of  the  earlier  eruption,  while  the  northern  peak  is  for  the  most  part 
normal;  but  it  also  shows  occasional  small  patches  resembling  its  southern 
neighbor,  and  there  seems  a  gradual  transition  from  one  to  the  other.  Fresher 
specimens  from  the  South  Peak  show  abundant  lustrous,  seemingly  black,, 
hornblendes  with  perfect  cleavage,  and  these  under  the  microscope  prove  to 


88  GEOLOGY  OF  THE  COMSTOCK  LODE. 

be  deep  brown  black -bordered  crystals.  There  is  no  green  hornblende  and 
thei-e  are  glass  inclusions.  In  short,  there  is  no  assignable  reason  for  sepa- 
rating this  rock  from  the  andesites.  There  are  many  other  localities  in  the 
District  where  the  propylitic  rocks  are  quite  as  puzzling  as  in  the  three 
described,  but  it  is  sufficient  to  state  that  they  Vrere  studied  with  equal  care 
and  with  similar  results. 

Conclusions  reached. — Field  obscrvations,  aided  by  microscopical  examina- 
tion, show  that  the  mineralogical  composition  and  the  structure  of  the  propy- 
lites  of  the  Washoe  District  in  their  original  state  were  identical  with  those 
of  certain  fresh  rocks  found  in  the  same  region,  namely,  granular  diorite, 
dioritic  porphyry,  diabase,  hornblende-andesite,  and  augite-andesite.  The 
great  and  misleading  similarity  of  the  propylites  to  one  another  is  due  not 
to  original  constitution,  nor  to  their  geological  relations,  but  to  the  identity 
of  the  decomposition  processes  to  which  they  have  all  been  subjected.  The 
failure  to  detect  the  lithological  relations  of  these  rocks  arose  principally 
from  a  confusion  between  green  hornblende  and  the  green  and  dichroitic, 
but  uniaxial,  minerals  grouped  under  the  term  chlorite;  but  a  neglect  to 
give  due  weight  to  evidences  of  pseudomorphism,  partial  devitrification 
and  other  phenomena  of  decomposition,  materially  aided  in  obscuring  the 
true  nature  of  the  supposed  rock-species. 

Causes  of  error. — It  appcars  to  mc  by  no  means  superfluous  to  consider  how 
so  keen  an  observer  as  Baron  v.  Richthofen  came  to  regard  propylite  in  the 
Washoe  District  as  an  independent  rock-species,  and  as  a  volcanic  of  Ter- 
tiary age;  and  while  I  have  no  authority  for  my  suggestions,  I  offer  the  fol- 
lowing explanation.^  Baron  v.  Richthofen  regarded  Mount  Davidson  as 
syenite  and  the  visible  plagioclases  as  accessory.  The  rock  does  indeed 
more  nearly  resemble  ordinary  syenite  in  its  general  appearance  than  ordi- 
nary diorite,  and  the  error  was  never  detected  until  Professor  Zirkel  exam- 
ined it  microscopically.  In  the  porphyritic  diorites  v.  Richthofen  saw  a 
plagioclase  rock,  but  the  triclinic  character  of  the  feldspars  in  the  porphyry 
aroused  no  known  doubt  in  his  mind  as  to  those  of  the  mass  of  Mount 
Davidson.     Porphyritic  syenites  are  very  rare,  while  the  relation  of  the 

'  It  would  be  superflnona  to  remind  geologists  that  in  1865  the  science  of  microscopical  lithology 
■was  undeveloped. 


PEOPYLITE.  89 

diorite-porphyries  of  Washoe  to  the  granitoid  diorite  is  peculiar.  Any 
but  a  very  thorough  inspection  would  lead  to  the  belief  that  the  porphyries 
are  younger  than  the  granular  diorite.  v.  Richthofen  had  reason  to  suppose 
that  Mount  Davidson  was  post-Jurassic,  and  the  plagioclase  porphyries  were 
therefore  in  his  eyes  younger  than  that  period,  and  older  than  the  andesites 
which  cap  the  range.  As  I  have  endeavored  to  show,  the  dioritic  porphy- 
ries, when  in  a  certain  stage  of  decomposition,  are  scarcely  distinguishable 
from  the  thoroughly  crystalline  andesites,  when  the  latter  are  also  somewhat 
decomposed.  These  v.  Richthofen  found  at  considerable  distances  from  his 
"syenite,"  and  so  associated  with  Tertiary  rocks  as  to  prove  them  members 
of  that  series.  Tertiary  leaves  were  also  discovered  in  similar  rock  at  no 
great  distance.  These  wackenitic  andesites,  too,  stood  in  such  a  relation  to 
the  fresher  rocks  that  they  appeared  to  precede  them,  and  the  chain  of  proof 
seemed  complete  of  a  pre-andesitic  Tertiary  rock.  The  extension  of  the 
propylite  to  the  mines  was  natural  and  easy. 

If  propylite  were  older  than  andesite,  where  should  we  look  for  it  but 
in  depth?  And  if  there  was  no  distinct  lithological  reason  assignable  for 
pronouncing  the  underground  rock,  mostly  in  the  last  stages  of  decomposi- 
tion, identical  with  that  on  the  surface,  there  was  next  to  no  reason,  macro- 
scopically  speaking,  for  supposing  it  different.  This  mistake  having  once 
been  committed,  I  do  not  believe  it  could  ever  have  been  corrected  in  ojopo- 
sition  to  even  a  far  less  weighty  authority  than  Baron  v.  Richthofen,  had 
not  fresher  rocks  been  opened  up  by  the  extensive  lower  workings,  and  had 
the  microscope  not  been  sought  as  an  auxiliary.  That  an  association 
between  propylites  and  mineral  veins  should  have  been  observed  is  natural, 
for  in  mineralized  districts  we  expect  general  decomposition. 

Propylites  from  other  di.tricts. — By  the  courtcsy  of  the  gcologists  of  the  Fortieth 
Parallel  Survey,  I  have  been  permitted  to  examine  the  specimens  and  slides 
from  all  the  localities  laid  down  in  their  publications  as  propylite.  Captain 
Button,  too,  has  kindly  furnished  me  with  specimens  and  slides  from  his 
propylite  localities  in  Utah.  Rocks  which  are  indeterminable  in  the  field 
are  very  apt  to  give  uncertain  evidence  under  the  microscope,  and  as  all 
propylites  are  decomposed,  I  do  not  feel  absolute  confidence  in  my  deter- 
minations of  the  propylites  occurring  outside  of  the  district  which  forms  the 


90  GEOLOGY  OF  THE  COMSTOOK  LODE. 

subject  of  this  paper.  Nevertheless,  I  have  given  my  notes  upon  them  in 
the  section  containing  the  "Detailed  description  of  slides."  There  appear 
to  be  fairly  good  grounds  for  the  determinations  there  suggested,  and  the 
specimens  seem  to  offer  no  evidence  even  approximately  suj0ficient  for  the 
establishment  of  a  new  rock-species. 

-  No  propyiite  yet  found  in  the  United  States. — Thc  term  propylite  might  bc  retained 
to  express  a  certain  macroscopical  appearance  and  certain  chemical  changes, 
just  as  we  still  speak  of  serpentine  without  denying  its  secondary  character. 
But  a  better  name,  and  an  older  one,  already  exists  for  this  very  thing,  for 
the  terms  greenstone  and  greenstone-trachyte  designate  rocks  in  every  way 
similar.  Considered  as  its  originator  intended  it,  as  a  pre-andesitic  Tertiary 
rock,  I  feel  no  hesitation  in  asserting  that  nothing  answering  to  its  defini- 
tion has  as  yet  been  proved  to  exist  in  the  United  States.^ 

'  European  propylites. — The  investigation  of  American  propylites  described  in  this  report  was  Carried 
out  entirely  without  reference  to  the  oj^inion  of  European  lithologists  regarding  the  Transylvanian 
rocks.  American  geologists  who  have  not  followed  the  subject  closely  may  be  interested  to  learn,  how- 
ever, that  the  tendency  of  opinion  in  Europe  is  strongly  against  the  independence  of  this  rock-species. 
Dr.  C.  Doelter  upholds  it  in  a  paper  "  Ueber  das  Vorkommen  des  Propylits  in  Siebenbiirgen.  Verhandl. 
der  k.  k.  Geolog.  Keichsanstalt,"  1875,  p.  27.  In  reviewing  this  paper  in  the  "Neues  Jahrbuch  fiir  Min- 
eralogie,"  etc.,  187i),  p.  648,  Professor  Rosenbusch  incidentally  considers  Baron  v.  Richthofen's  descrip- 
tion and  Professor  Zirkel's  views,  and  states  his  own  conclusions  as  follows  (translated) : 

"  The  reviewer,  in  common  with  all  other  investigators,  willingly  recognizes  the  peculiar  green- 
stone habitus  of  the  so-called  propylites ;  their  Tertiary  age,  which  in  many  cases  must  be  further  and 
more  sharply  determined,  being  assumed.  Since  similar  changes  in  habitus  occur  in  many  other  series 
of  rocks,  however,  he  does  not  feel  himself  compelled  to  accord  propylite  an  independent  position,  but 
rather  to  regard  it  as  a  mere  pathological  variety  of  qnartzose  or  quartzless  hornblende-andesites,  or  of 
the  augite-andesites,  as  the  case  may  be." 

Professor  vom  Rath  has  published  a  paper  in  the  "  Sitzungsberichte  der  Neiderrheinischen  Gesell- 
schaft  in  Bonn,"  vol.  35,  1878,  p.  26,  in  which  he  expresses  a  very  positive  opinion  that  the  so-called 
propylite  of  Scheranitz  is  diabase,  and  has  no  relation  to  the  andesites  of  the  neighborhood.  He  assorts 
that  this  diabase  has  a  very  different  look  macroscopically  and  microscopically  from  andesites,  but  it  is 
to  be  regretted  that  he  does  not  give  the  dififerences  in  sufficient  detail  to  enable  readers  to  judge  for 
themselves.  Prof.  J.  Szab6  has  read  a  paper  before  the  Hungarian  Geological  Society,  which  is 
reported  in  the  Verhandluugen  der  k.  k.  Geolog.  Reichsanstalt,  1879,  Literatumotizen,  p.  17.  In  this 
paper  Professor  Szab6  maintains  that  various  eruptive  rocks  and  even  sedimentaries  have  been  altered 
to  what  is  called  greenstone  by  solfataric  action  at  Schemnitz,  and  he  concludes  with  the  following 
statement  (translated) :  "  There  is  no  greenstone-trachyte  formation  proper  in  a  geological  sense  ;  there 
has  never  been  an  independent  propylite  eruption."  I  infer  that  the  conditions  in  Schemnitz  are  sub- 
stantially similar  to  those  in  Washoe. 


DETAILED  DESCRIPTION  OF  SLIDES.  91 


Section    4.     (Chapter  III.) 
DETAILED  DESCEIPTION  OF  SLIDES. 

Reasons  for  this  section. — In  view  of  the  considei'able  alterations  proposed  in 
the  classification  of  the  Washoe  rocks,  it  appears  proper  to  submit  detailed 
descriptions  of  a  sufficient  number  of  slides  to  enable  lithologists  to  judge 
whether  the  methods  employed  in  the  determinations  are  correct  and  the 
grounds  upon  which  distinctions  have  been  drawn  sufficient.  Nearly  but 
not  quite  all  the  statements  made  in  the  foregoing  sections  of  this  chapter 
concerning  the  microscopical  character  of  the  rocks  may  be  substantiated 
from  these  slides.  It  was  considered  that  further  descriptions  were  need- 
less and  would  be  burdensome. 

Determination  of  feldspar. — The  feldspars  have  becH  determined  optically  ac- 
cording to  the  rules  laid  down  by  Messrs.  Fouqud  and  Levy.^  This  method 
is  very  tedious,  and  is,  properly  speaking,  applicable  only  to  the  determina- 
tion of  the  most  basic  feldspar  present;  but  by  applying  it  to  a  great  num- 
ber of  cases  the  microscopist  is  able  to  satisfy  himself  of  the  prevailing 
feldspars  as  well,  and  in  this  respect  it  appears  to  me  more  satisfactory  than 
the  determination  of  isolated  feldspar  fragments  by  their  specific  gravity. 
In  two  cases  M.  Thoulet's  method  has  been  employed.  Professor  Szab6's 
method  has  not  been  attempted.^ 

An  explanation  of  the  method  of  reference  to  the  slides  by  a  system  of 
coordinates  in  millimeters,  referred  to  the  upper  left-hand  corner  of  the  glass, 
will  be  found  in  the  description  of  the  lithological  illustrations,  page  145. 


GRANITE. 

Slide  460.    Close  to  Red  Jacket  mine. 

Typical  granite. — This  is  a  moderately  fine-grained  gray  micaceous  granite. 
The  slide  shows  besides  orthoclase,  quartz,  and  mica,  a  few  plagioclases, 

'  Miueralogie  Micrographique,  1879.     So  far  as  I  know  this  method  was  first  suggested  by  Prof. 
R.  PmiipoUy,  Proc.  Amer.  Acad.,  Vol.  XIII.,  p.  258. 

^  Tests  by  this  method,  subsequently  made,  are  described  on  p.  40.'5,  et  seq. 


92  GEOLOGY  OF  THE  COMSTOGK  LODE. 

magnetite,  and  some  accessory  minerals.  The  structure  is  typically  granit- 
oid, none  of  the  principal  minerals  showing  either  perfect  crystalhne  out- 
lines or  microlitic  development.  The  orthoclase  is  for  the  most  part  trans- 
parent, and  in  many  cases  shows  good  cleavages,  which  are  usually  parallel  to 
the  extinctions.  The  plagioclases  show  very  narrow  stripes  and  no  angles  of 
extinction  exceeding  those  of  oligoclase.  The  quartz  contains  abundant  liquid 
inclusions,  many  with  moving  bubbles.  The  mica  shows  the  interference 
figure  of  biotite,  and  is  of  course  brown  and  highly  dichroitic.  A  portion 
of  the  biotite  appears  "bleached"  to  a  lighter  brown,  and  other  fragments 
are  converted  into  chlorite.  A  few  particles  of  epidote  are  visible,  forming 
from  the  chlorite.  The  iron  ore  is  evidently  magnetite,  occurring  mostly  in 
quadrangular  forms,  and  being  accompanied  by  hematite.  There  is  also  a 
considerable  amount  of  titanite,  which  in  some  cases  takes  the  form  of  per- 
fect rhombs,  with  an  angle  of  somewhat  less  than  140°.  It  shows  the 
cleavages,  the  rough  surface,  high  refraction,  and  dull  colors  between  crossed 
Nicols,  appropriate  to  sphene.  There  are  many  minute  zircons,  and  some 
ordinary  apatites.  The  slide  contains  two  patches  of  a  somewhat  highly 
refracting,  nearly  colorless,  slightly  yellowish,  mineral,  one  of  which  seems 
to  be  of  an  imperfect  hexagonal  outline,  and  the  other  nearly  square.  They 
show  a  rippled  surface,  such  as  is  often  seen  on  augite.  They  remain 
dark  between  crossed  Nicols,  and  give  no  interference  figure.  The  mineral 
shows  cracks,  some  of  which  are  irregular;  others  seem  referable  to  an  im- 
perfect rhombohedral  cleavage.  All  these  properties  suggest  sodalite.  This 
mineral,  however,  has  been  noticed,  I  believe,  among  the  older  massive 
rocks  only  in  syenite,^  and  in  combination  with  elseolite  and  zircon.*  As 
zircon  is  plentiful  in  this  slide,  I  carefully  looked  for  elseolite.  If  present 
at  all  it  must  be  in  granitoid  crystals,  which  might  be  mistaken  for  ortho- 
clase. Many  such  are  cut  nearly  at  right  angles  to  an  optical  axis,  but  I 
failed  to  find  one  such  which  gave  the  interference  figure  of  a  uniaxial  min- 
eral. 

1  As  the  name  is  now  usually  nnderstood.    In  Dana's  Mineralogy  the  quartzless  mica-orthoolase 
rocks  are  stUl  termed  granite. 


DETAILED  DESCRIPTION  OF  SLIDES.  93 

GRANULAR  DIORITE. 
Slide  213.    Bullion  Eavine,  at  Water  Company's  flume. 

Typical  diorite  with  green  fibrous  hornblende. Thls    IS    the    tjpical    dioHte  Of    MoUIlt 

Davidson.  Macroscopically,  it  is  gi'ay  in  color  and  granitic  in  structure. 
The  sUde  shows  that  it  is  composed  of  a  mass  of  crystalline  grains,  filling 
the  whole  space  and  without  the  most  distant  approach  to  a  porphyritic 
structure.  It  contains  triclinic  feldspar,  fibrous  hornblende,  quartz,  magnet- 
ite, a  few  fragments  of  mica,  and  a  number  of  accessory  minerals.  The 
hornblende  is  present  only  in  fibrous  crystalline  masses  and  patches,  which 
seem  to  have  crystallized  after  the  feldspar.  Many  of  the  masses  of  horn- 
blende are  cut  at  right  angles  to  the  main  axis,  and  show  excellent  cleavages 
at  the  characteristic  angles.  It  is  strongly  dichroitic,  giving  tints  varying 
from  buflf  to  sea-green.  It  polarizes  with  great  brilliancy,  showing  the 
whole  range  of  prismatic  colors.  The  angles  of  extinction  observed  reached 
20°.  lu  parts  of  the  slide  the  hornblende  is  decomposed,  the  products 
being  chlorite,  epidote,  quartz,  and  calcite.  The  disposition  of  the  original 
mineral  is  so  irregular  that  the  process  of  decomposition  cannot  be  studied 
to  advantage. 

The  feldspars  seem  to  be  without  exception  polysynthetic  plagioclases. 
The  twin  striations  are  irregular  in  width,  but  very  continuous  and  sharply, 
defined.  The  angles  of  extinction  of  the  twins,  which  extinguish  light  at 
equal  inclinations  to  the  plane  of  the  Nicols,  are  large.  Very  many  such 
were  observed  to  exceed  20°,  and  one  or  two  reach  29°.  The  feldspar  is 
therefore  in  the  main  labradorite,  and  I  saw  no  indications  of  the  presence 
of  any  other  feldspar  species.  There  are  no  untwinned  feldspars  or  feld- 
spathic  microlites.  Besides  the  twins  following  the  law  of  albite,  there  are 
many  instances  of  additional  periclinic  twinning.  In  several  crystals  there 
is  well-developed  zonal  structure.  The  feldspars  are  for  the  most  part  very 
free  from  inclusions  of  any  kind,  and  are  clear  and  transparent. 

Many  grains  of  quartz  are  present,  but  I  observed  no  crystal  faces. 
The  quartzes  are  full  of  fluid  inclusions,  some  of  them  dihexahedral.  One 
of  these  is  so  large  that  the  movement  of  the  bubble  can  be  cleai-ly  seen 
with  a  magnifying  power  of  60  diameters.     The  bubbles  of  these  inclusions 


94  GEOLOGY  OF  THE  COMSTOCK  LODE. 

do  not  disappear  upon  heating  the  shde  to  40°  C  on  Vogelsang's  table,  and 
are  therefore  probably  aqueous. 

There  is  a  considerable  quantity  of  magnetite  in  this  slide,  characterized 
by  its  square  outlines  and  opacity.  I  observed  no  titanic  iron.  A  few 
crystals  of  apatite  appear  under  the  microscope,  rather  fewer  than  is  usual 
in  the  rocks  of  the  Disteigt.  They  are  colorless,  and  contain  no  determinable 
inclusions.  There  are  many  minute  zircons  recognizable  by  their  high  refrac- 
tion, brilliant  polarization,  and  by  their  crystal  form  (the  eight-sided  prism, 
terminated  by  the  fundamental  pyramid).  One  or  two  fragments  of  mica 
appear  in  the  slide — e.  ^.,  at  21-21.  -There  are  also  a  number  of  irregular 
fragments  of  a  mineral  which  can  scarcely  be  anything  but  titanite.  It 
shows  an  uneven  surface,  brown  color,  perceptible  dichroism,  and  high 
refractive  index.  In  polarized  light  it  is  only  feebly  chromatic.  Plate  IV., 
Fig.  25,  shows  a  characteristic  portion  of  this  slide. 

Slide  413.     Union  Shaft,  2,625  feet  from  surface. 

Dark  dioritc  with  some  brown  hornblende. — Macroscoplcally  this  is  a  vcry  dark  rock, 
highly  charged  with  scales  of  hornblende.  It  reminds  one  of  freshly  fract- 
ured "No.  1"  pig  iron.  Under  the  microscope  it  is  seen  to  be  composed 
essentially  of  triclinic  feldspar  and  hornblende,  both  minerals  having  con- 
solidated nearly  at  the  same  time.  A  few  grains  of  quartz,  and  an  insig- 
nificant amount  of  colorless  apatite,  complete  the  list  of  components. 

The  hornblende  is  in  part  of  a  brown  tint,  very  slightly  tinged  with 
green;  in  part  it  is  of  a  light  and  vivid  blue-green  color.  Many  of  the 
hornblende  crystals  show  both  colors ;  the  green  variety  occurring  along 
the  edges  and  cleavages,  and  sometimes  leaving  only  small  irregular  patches 
of  the  brown  mineral  surrounded  by  the  green.  The  structure  of  the  two 
varieties  is  distinctly  different.  The  brown  mineral  shows  excellent  cleav- 
ages, but  no  tendency  to  fibration.  In  the  green  portions  of  the  same  indi- 
viduals the  hornblende  seems  to  be  composed  of  minute  fibers,  but  the 
tesselated  appearance  of  the  cross-sections  is  nearly  obliterated.  In  fact  all 
the  appearances  are  such  as  accompany  a  distinct  alteration  in  mineral  char- 
acter.     The  brown  hornblende  is  as  usual   very  strongly  dichroitic;  the 


DETAILED  DESCKIPTION  OF  SLIDES.  95 

green  is  less  so.     On  the  other  hand,  the  green  mineral  polarizes  in  colors 
of  the  utmost  brilliancy,  like  those  of  the  preceding  slide. 

The  hornblendes  contain  a  vast  number  of  included  microlites  of  a 
black,  wholly  opaque  mineral,  crystallizing  in  needles  and  long  pointed 
scales,  which  can  scarcely  be  anything  but  ilmenite  or  hematite.  These 
microlites  are  arranged  in  certain  planes  of  the  hornblende  crystals,  viz: 
perpendicular  to,  and  parallel  to  the  base.  In  sections  nearly  parallel  to  the 
vertical  axis  no  further  regularity  is  perceptible,  but  cross-sections  show 
that  they  are  also  parallel  to  the  prismatic  faces  and  to  the  clinopinacoid 
The  distances  from  these  faces  are  wholly  irregular,  and  the  effect  is  there- 
fore merely  that  the  microlites  form  with  one  another  angles  of  nearly  60°. 
It  is  noteworthy  that  just  the-faces  most  usually  found  in  microscopic  horn- 
blendes are  the  ones  emphasized  by  the  position  of  these  minute  bodies. 
The  same  microlites  also  occur  in  the  feldspars,  in  which,  too,  their  distri- 
bution seems  to  be  governed  in  part  by  some  crystallographic  law,  but  what 
one  is  not  evident  from  this  slide.  These  microlites  are,  for  the  most  part, 
entirely  unaltered  in  the  brown  hornblende,  while  in  the  green  they  are 
replaced  in  part  by  very  fine  transparent  yellowish  crystalline  grains.  In 
some  places  the  black  and  the  transparent  inclusions  are  continuous  with 
one  another,  and  everywhere  the  disposition  of  the  latter  is  j^recisely  that 
of  the  former.  In  fact  a  narrow  inspection  does  not  leave  a  doubt  that  the 
opaque  microlites  are  decomposed  into  a  transparent  mineral.  The  minute 
size  of  the  grains  found  does  not  permit  of  absolute  determination ;  but  the 
product  of  decomposition  is  doubly  refracting,  possesses  a  high  index  of 
refraction,  is  slightly  dichroitic,  and  seems  to  polarize  in  rather  feeble  colors. 
The  only  familiar  minerals  which  it  recalls  are  titanite  and  epidote,  and  the 
probabilities  are  that  it  is  sphene. 

In  one  portion  of  the  slide  is  a  mass  of  a  nearly  colorless  substance, 
slightly  tinged  with  green,  which  seems  to  be  totally  isotropic.  Under 
crossed  Nicols  it  remains  absolutely  dark,  and  when  the  quartz  plate  is 
introduced,  and  the  Nicols  are  adjusted  to  the  teinte  sensible,  no  change 
whatever  in  the  shade  is  perceptible  on  revolving  the  slide  This  is  one  of 
the  substances  grouped  under  the  term  "chloritic  constituents,"  but  it  does 
not  appear  to  be  certainly  identical  with  the  ordinary  product  of  the  decom- 


96  GEOLOGY  Ol^^  THE  COMSTOGK  LODE. 

position,  of  hornblende.  Embedded  'v.  it  are  numerous  small  grains  and 
microlites,  which  extinguish  light  at  a  large  angle  to  the  plane  of  the  Nicols. 
They  are  arranged  at  angles  of  about  60°,  and  it  appears  to  me  that  the 
object  must  be  supposed  to  be  a  decomposed  hornblende,  filled  with  micro- 
lites of  the  mineral  which  results  from  the  decomposition  of  the  black 
microlites.  This  opinion  is  strengthened  by  the  occurrence  of  a  number  of 
intermediate  stages,  as  they  seem  to  be,  between  fresh  hornblende  and  the 
last  mentioned  chloritic  mass.  As  the  black  microlites  alter,  the  hornblendes 
become  in  some  cases  grayish  and  less  and  less  pellucid,  not  apparently 
from  want  of  transparency  on  the  part  of  the  minerals,  but  through  irregular 
refraction  of  light. 

The  feldspar  is  undoubtedly  for  the  most  part  labradorite,  many  of  the. 
finely  twinned  crystals  showing  angles  of  extinction  of  nearly  30°  on  each 
side  of  the  twinning  jjlane.  I  see  no  evidence  of  the  presence  of  any  other 
feldspar.  There  are  a  few  grains  of  quartz,  which  contain  some  liquid  in- 
clusions. The  apatites  are  few  in  number,  colorless,  and  in  no  way  remark- 
able.    I  detected  no  other  minerals  in  the  slide. 

Slide  81.     Utah,  1,950. 

Gray  diorite  with  brown  hornblende. — This  is  the  freshest  dioHte  iu  the  coUectiou, 
the  feldspar  being  as  transparent  as  it  ordinarily  is  in  andesite.  Unfortu- 
nately the  slide  is  not  thin.  The  principal  difi"erence  between  this  and 
slide  413  is  that  the  majority  of  the  hornblendes  are  brown,  many  of  them 

without  a  tinge  of  green. 

« 
Slide  361.     Savage  1,300.    North  drift,  about  310  feet  in. 

Micaceous  granular  diorite. — lu  this  rock,  whicli  is  uot  a  porphyrite,  but  gran- 
ular, the  hornblende  has  been  almost  wholly  replaced  by  biotite,  which  is 
of  the  usual  structure,  and  gives  an  interference  figure  nearly  like  that  of 
a  uniaxial  mineral.  The  slide  contains  much  quartz  and  many  beautifully 
sharp  zircons.     In  other  respects  it  is  similar  to  slide  213. 

Slide  291.     Ghollar  1,700 ;  1,425  feet  west  of  Combination  shaft. 

Diorite  containing  tourmaline,  etc. — This  is  a  diorito  of  the  grauulsr  crystalHue 
type,  but  of  a  very  quartzose  variety.     The  quartzes  contain  innumerable 


DETAILED  DBSCEIPTIOK  OF  SLIDES.  97 

fluid  inclusions,  many  of  them  of  unusually  large  size.  Some  are  dihexahe- 
dral  in  shape  ;  the  bubbles  of  the  smaller  ones  are  active,  and  some  contain 
excellent  salt  cubes.  The  proportion  of  salt  to  water  seems  to  be  very  high; 
for  on  heating  the  slide  to  about  70°  C,  the  only  effect  produced  was  to 
round  the  edges  of  the  cubes.  The  bubbles  did  not  grow  perceptibly 
smaller  at  this  temperature. 

The  slide  is  further  remarkable  for  containing  what  appears  to  be 
tourmaline.  One  small  patch  dichroizes  between  black  and  clear  brown. 
The  mineral  exhibits  scarcely  any  structure,  but  there  are  traces  of  what 
appear  to  be  cleavage  cracks  parallel  to  the  direction  of  extinction.  No 
distinct  interference  figtire  could  be  obtained.  The  lack  of  structure  and 
the  absolute  extinction  of  the  ordinary  ray  seem  to  separate  this  substance 
from  hornblende ;  to  mica  it  bears  no  resemblance. 


POEPHYKITIO   DIOEITE. 

Slide  421.    Center  of  Cedar  Hill  Ridge. 

Fresh  porphyry. — Thc  mass  of  porphyritc  forming  Cedar  Hill  is  very  uneven 
in  composition,  and,  for  the  most  part,  greatly  decomposed.  Near  the  high-- 
est  portion,  however,  is  a  small  quantity  of  a  comparatively  fine-grained 
variety,  which,  from  one  of  the  accidents  so  common  in  regions  of  decom- 
position, has  escaped  nearly  unaltered.  Macroscopically  it  is  a  dark,  leaden- 
gray  rock,  rather  fine  in  texture,  and  exhibiting  porphyritical  crystals  of 
feldspar  and  hornblende.  Under  the  microscope  it  is  seen  that  these  min- 
erals are  separated  out  in  a  groundmass  of  tolerably  fresh  feldspar  micro- 
lites,  and  magnetite,  to  which  the  dark  color  of  the  rock  is  due.  Numerous 
colorless  apatites  form  the  only  other  prominent  mineral  ingredient.  The 
hornblendes  are  almost  wholly  undecomposed.  They  are  of  a  slightly 
greenish-brown  color  and  fairly  well-crystallized.  Most  of  this  mineral 
occurs  in  crystals  of  large  size,  but  there  are  a  few  minute  crystals  and 
crystalline  fragments  interspersed  through  the  groundmass.  The  horn- 
blende is  dense,  though  in  many  cases  the  cleavages  are  well  developed, 

and  one  crystal  even  contains  fluid  inclusions  (10-24  J).     There  is  no  tend- 
7  c  L 


98  GEOLOGY  OF  THE  COMSTOCK  LODE. 

ency  to  zonal  structure  in  this  slide,  but  several  of  the  hornblendes  are 
twinned  according  to  the  ordinary  law.  Decomposition  has  set  in  to  a 
slight  extent ;  and  in  one  or  two  cases  the  degeneration  into  chlorite  may 
be  observed  starting  from  the  cleavage  fissures  of  the  parent  mineral.  Where 
the  masses  of  chlorite  have  reached  any  considerable  size,  particles  of  epidote 
have  developed  near  their  centers.  In  a  large  proportion  of  the  hornblendes 
occur  inclusions  of  the  same  kind  mentioned  under  slide  413.  A  group  of 
these  microlites  is  shown  in  Fig.  21,  Plate  III.  Their  disposition  is  the  same 
as  in  slide  413,  but  this  section  contains  nothing  which  throws  light  on  their 
natiire. 

There  are  numerous  good-sized  but  rounded  plagioclases  in  this  slide. 
Those  which  show  an  approximately  equal  angle  of  extinction  on  each  side 
of  the  twinning  plane,  give  angles  of  extinction  which,  in  some  cases,  con- 
siderably exceed  20°;  no  untwinned  microlites  were  observed,  and  the 
feldspar  is  probably  labradorite.  The  feldspars  contain  a  few  fluid  inclusions 
of  apparently  primitive  character,  and  are  pierced  by  numerous  apatite 
needles.  One  or  two  fragments  of  hornblende  are  inclosed  in  feldspars, 
but  for  the  most  part  the  feldspars  are  wholly  free  from  that  mineral. 

The  groundmass  consists  mainly  of  feldspar  microlites  and  granules,  and 
traces  of  fluidal  structure  are  perceptible.  An  abundance  of  magnetite  is 
recognizable  as  such  from  its  crystal  form ;  and  associated  with  and  pene- 
trating it  are  many  colorless  apatites.  The  slide  also  contains  one  poorly 
developed  zircon.  There  is  further  a  small  amount  of  chlorite  and  epidote. 
Most  of  the  former  is  concentrated  in  an  excellent  vein. 

Except  in  the  matter  of  inclusions,  this  rock  bears  a  strong  resemblance 
to  an  andesite ;  its  groundmass,  however,  is  less  microlitic  and  the  porphyritic 
feldspars  have  not  the  sharp  development  almost  invariably  observable  in 
andesites.  Its  occurrence  as  a  mass  little  more  than  a  foot  cube,  embedded 
in  porphyritic  diorites  of  an  ordinary  variety,  forbids  the  supposition  that  it 
is  a  volcanic  rock.     A  portion  of  the  slide  is  shown  in  Fig.  26,  Plate  IV. 

Slide  278.    Ophir  Eavine,  south  side. 

A  second  fresh  porphyry. — This  rock  strongly  rescmblos  421  in  most  respects, 
but  the  hornblendes  are  noteworthy.     They  are  unusually  solid,  often  show- 


DETAILED  DESCEIPTION  OF  SLIDES.  99 

ing  scarcely  a  trace  of  cleavage.  Indications  of  zonal  structure  are  visible; 
i.  e.,  the  exterior  layer  of  the  mineral  exhibits  a  somewhat  different  texture 
from  the  remaining  mass.  The  polarization  of  these  hornblendes  is  remark- 
ably brilliant,  quite  equalling  that  of  ordinary  augite.  Many  of  the  crys- 
tals are  twinned,  one  of  them  (14-23)  being  polysynthetic.  A  crystal  of 
considerable  size  is  divided  into  halves  of  identical  orientation  by  a  narrow 
layer  of  the  mineral  in  a  reversed  position. 

In  one  part  of  the  &lide  (13-27)  are  some  minute  scales  of  epidote 
which  appear  to  represent  the  clinopinacoid,  limited  by  the  base,  the  ortho- 
pinacoid,  and  the  positive  hemidome.  The  direction  of  extinction  is  sensibly 
perpendicular  to  the  orthopinacoid.  The  same  form  of  epidote  is  found  in 
other  slides,  e.  g.,  in  371  at  17^-19. 

Slide  252.     Sierra  Nevada,  1450.    North  drift  289  feet  north. 

Partially  decomposed  dioritic  porphyry. — This  is  a  grayish-grecu  granitic-lookiug 
rock,  with  brilliant  hornblendes,  and  only  a  slight  apparent  tendency  to 
porphyritic  structure.  Under  the  microscope,  however,  it  is  seen  to  belong 
among  the  porphyritic  diorites.  The  feldspars  are  almost  opaque,  and  it  is 
with  some  difficulty  that  they  can  be  made  out  to  be  triclinic.  The  ground- 
mass  was  evidently  granular  when  fresh.  There  appears  to  have  been  a 
little  mica,  now  converted  to  chlorite  and  epidote.  The  hornblendes  are 
unusuall}^  interesting  because  present  in  all  stages  of  decomposition.  The 
fresher  ones  are  bright  brown,  without  black  borders,  and  solid  except  for 
the  well-marked  cleavages.  Other  crystals  seem  to  have  undergone  a  spe- 
cies of  fibration  in  the  direction  of  the  cleavages.  This  fibration  is  accom- 
panied by  the  presence  of  decomposition  products,  and  each  small  elongated 
cleavage  prism  seems  coated  with  secondary  minerals.  Other  hornblendes 
are  partially  converted  into  chlorite,  and  a  fine  example  is  illustrated  in  Plate 
II.,  Fig.  1 .  Still  others  have  passed  completely  into  epidote.  In  some  of 
the  partially  decomposed  hornblende  crystals  there  are  small  crystals  of 
pyrite. 

Slide  194.    McKibben  Tunnel,  480  feet  from  entrance. 

Decomposed  dioritic  porphyry. — In  hand  spccimcns  this  rock  is  greenish-gray, 
and  somewhat  porphyritic.     Under  the  microscope  it  is  seen  to  be  greatly 


100  GEOLOGY  OP  THE  COMSTOCK  LODE. 

decomposed,  but  not  in  such  a  manner  as  to  obscure  its  original  constitu- 
tion. When  fresh  it  consisted  essentially  of  well-developed  crystals  of  tri- 
clinic  feldspar  and  hornblende,  disposed  porphyritically  in  a  groundmass 
mainly  composed  of  feldspathic  grains.  A  little  mica,  a  small  amount  of 
black  ore  (probably  magnetite),  and  numerous  colorless  crystals  of  apatite, 
were  subordinate  mineral  ingredients. 

No  undecomposed  hornblende  now  remains.  It  has  been  replaced  by 
chloritic  material,  epidote,  quartz,  and  calcspar,  but  in  such  a  way  as  to 
leave  the  larger  portion  of  the  hornblende  crystal  outlines  undisturbed. 
All,  or  nearly  all,  the  hornblendes  seem  to  have  been  crystals  of  consid- 
erable size  and  sharp  definition,  and  there  is  nothing  to  indicate  that  they 
possessed  a  fibrous  structure.  Some  of  the  hornblende  crystal  outlines 
are  completely  filled  with  the  chlorite.  This  substance  sometimes  shows 
an  excessively  fine,  fibrous,  imperfectly  spherolitic  structure.  In  other 
cases  the  fibers  near  the  peripheries  of  former  hornblendes  are  arranged  at 
right  angles  to  the  crystal  face.  These  fibers  are  of  nearly  equal  length, 
and  they  form  a  zone  just  within  the  crystal  section.  The  chlorite  is  grass- 
green,  and  very  slightly  dichroitic,  varying  between  more  and  less  yellow- 
ish green  shades.  Between  crossed  Nicols  it  behaves  almost  like  an  isotropic 
substance  and  shows,  besides  black,  only  dark  purple  tints.  The  chlorite 
is  not  confined  to  the  hornblende  sections,  but  is  difPused  through  the  rock 
in  veins  and  patches.  It  also  occurs  in  narrow  borders  about  magnetite 
and  apatite,  as  if  these  minerals  had  mechanically  obstructed  its  move- 
ments. 

The  epidote  occurs  in  a  similar  way  both  without  and  within  the  horn- 
blende sections,  which  it  sometimes  wholly  and  sometimes  only  partly  fills. 
It  is  noteworthy  that  this  mineral  when  it  occurs  in  small  patches  is  usually 
finely  granular,  and  that  within  certain  limits,  the  larger  the  area,  the 
coarser  the  grain.  When,  as  is  often  the  case,  the  occurrences  are  wedge- 
shaped,  the  granulation  grows  coarser  from  the  point  to  the  base.  This 
seems  to  indicate  a  more  or  less  continuous  recrystallization  of  the 
mineral. 

The  relations  of  the  chlorite  and  epidote  in  this  slide  are  extremely 
interesting,  for  it  affords  abundant  proof  that  the  epidote  has  formed  at  the 


DETAILED  DESCRIPTION  OF  SLIDES.  '  101 

expense  of  the  chlorite.  This  is  well  illustrated  in  Figs.  6  and  7,  Plate  II., 
especially  in  Fig.  7,  where  the  growth  of  the  epidote  into  the  chlorite  is 
accurately  and  clearly  shown.  It  is  very  noticeable  that,  as  has  already 
been  mentioned,  the  chlorite  at  the  edges  of  the  hornblende  sections  fre- 
quently remains  undecomposed  longer  than  the  interior  mass.  The  behav- 
ior of  this  peculiarly  arranged  chlorite  seems  to  indicate  a  greater  density, 
and  consequentlj^  a  greater  resistance  to  decomposition,  than  is  possessed  by 
that  with  spherolitic  structure.  In  a  majority  of  cases  the  decomposition 
of  chlorite  into  epidote  begins  toward  the  center  of  the  section,  but  there 
are  many  exceptions.  It  is  probable  that  the  veins  and  patches  of  epidote 
not  connected  with  the  hornblende  sections  have  also  been  formed  from 
chlorite,  for  the  latter  appears  to  be  the  more  soluble  mineral.  There  is 
evidence  too,  from  other  slides  of  the  same  rock,  that,  as  decomposition 
proceeds,  the  chlorite  is  replaced  to  an  increasing  extent  by  epidote,  etc. 
The  chlorite  in  this  rock  also  decomposes  into  quartz,  calcite,  and  limonite. 
Whether  epidote,  too,  undergoes  the  same  decomposition  is  uncertain. 
Foinaaing,  as  it  does,  masses  of  irregular  granules  and  imperfect  prisms,  it 
would  be  difficult  to  .show  that  it  had  been  encroached  upon  in  any  given 
case  by  quartz  and  calcite,  and  had  not  formed  simultaneously  with 
them. 

There  is  no  augite  in  this  rock,  but  a  little  (5-23)  mica,  which,  like 
the  hornblende,  has  been  converted  into  chlorite  and  epidote.  The  feld- 
spars still  show  twin  striations,  but  are  considerably  decomposed,  and  under 
high  powers  the  mass  is  seen  to  be  porous  or  even  spongy.  Particles  of 
chlorite,  epidote,  quartz,  and  calcite  are  disseminated  through  the  feldspars. 
In  some  of  the  freshest  portions  fluid  inclusions  may  be  detected.  The  apa- 
tites are  all  colorless,  and  sharply  crystallized.  Fig.  18,  Plate  III.,  shows  a 
curious  case,  in  which  an  intrusive  bay  of  groundmass  has  reduced  an  apa- 
tite section  to  the  form  of  a  horseshoe.  There  is  a  considerable  amount  of 
pyrite  in  this  rock  (which  occurs  near  ore),  but  only  a  trifling  amount  of 
magnetite.  The  groundmass  shows  gray,  semi-opaque  markings,  not  dis- 
similar to  stippling.  This  appearance  is  caused  in  part  by  particles  of  cal- 
cite, etc.,  but  close  examination  shows  that  it  is  largely  due  to  the  spongy 
structure  mentioned  above.  > 


102  GEOLOGY  OP  THE  COMSTOCK  LODE. 

Slide  197.    McKibben  Tunnel,  488  feet  from  entrance. 

Decomposed  dioritic  porphyry. — This  sHdc  is  from  the  SRiue  bodj  of  porphyrite 
as  194,  which  it  greatly  resemhdes.  Fig.  10,  Plate  III.,  from  this  slide,  shows 
a  mass  of  chlorite  bounded  by  the  outlines  of  a  former  hornblende.  A 
portion  of  this  chloiite  has  been  converted  into  a  mixture  of  quartz  and 
calcite,  accompanied  by  limonite.  This  pseudomorph  seems  to  prove  that 
the  survival  of  a  border  of  chlorite  at  the  outer  edge  of  the  hornblende  sec- 
tion accompanies  the  decomposition  of  chlorite  into  quartz,  etc.,  as  well  as 
the  change  into  epidote. 

Slide  199.    McKibben  Tunnel,  488  feet  from  entrance. 

This  slide,  from  the  same  specimen  as  197,  contains  a  fine  hornblende 
section  completely  changed  into  epidote.  In  this  case  the  formation  of 
epidote  appears  to  have  started  from  points  near  the  edge.  It  is  shown  in 
Fig.  9,  Plate  III. 

Slide  281.    Head  of  Ophir  Eavine. 

Decomposed  diorite-porphyry. — This  Tock  strongly  rcsemblcs  that  from  the  Mc- 
Kibben Tunnel  both  macroscopically  and  microscopically.  It  forms  very 
extensive  croppings,  different  portions  of  which  vary  greatly  in  degree  of 
decomposition  and  appearance.  Where  most  decomposed  it  is  reduced  to  an 
almost  uniform  dull  green  color,  but  in  the  freshest  portions  it  is  granular, 
greenish  gray  in  tint,  displays  its  feldspars  and  altered  hornblendes  in 
marked  contrast,  and,  in  short,  betrays  its  dioritic  character.  Under  the 
microscope  this  slide  shows  the  original  constituents  to  have  been  feldspar, 
well  crystallized  hornblende,  some  augite,  magnetic  iron,  and  apatite. 

The  hornblende  has  been  completely  decomposed,  and  comparatively 
little  chlorite  remains  within  the  hornblende  sections,  which  are  mainly  filled 
with  epidote.  A  definite  geometrical  relation  is  noticeable  here,  as  in  slide 
194,  between  the  outlines  of  the  hornblendes  and  the  progress  of  the 
decomposition.  Many  of  the  outlines  of  hornblende  sections  are  occupied 
towards  the  center  by  a  mass  of  epidote,  between  which  and  the  periphery 
is  a  band  mainly  filled  by  quartz.  Either,  then,  the  chlorite  has  been  decom- 
posed from  the  center  into  epidote,  and  simultaneously  from  the  exterior 
into  quartz;  or  the  epidote,  after  replacing  the  chlorite,  has  been  decom- 


DETAILED  DESCRIPTION  OP  SLIDES.  103 

posed  from  the  periphery  of  the  hornblende  section.  The  former  supposi- 
tion is  altogether  the  more  probable.  A  portion  of  the  epidote  does  not 
show  the  usual  crystalline  structure,  but  forms  a  mass  of  small  grains  or 
scales,  of  which  so  many  are  superimposed  upon  one  another  in  the  thick- 
ness of  the  section,  as  to  present  perfect  aggregate  polarization;  indeed  it 
is  difficult  to  detect  a  difference  between  these  masses  in  polarized  light  and 
natural  light.  The  change  of  the  edges  of  the  hornblendes  to  quartz  has 
been  accompanied  by  the  separation  of  minute  particles  of  a  whitish  opaque 
material  of  unknown  character,  and  further  by  the  formation  of  black 
opaque  particles  which  can  hardly  be  anything  else  than  hematite  or  mag- 
netite. These  particles  are  arranged  in  lines  parallel  to  the  crystal  edges, 
and  now  surround  many  of  the  interior  masses  of  epidote  with  a  black 
border.  This  is  interesting  as  evidence  that  the  black  border  of  decomposing 
hornblendes  is  sometimes  a  secondary  formation. 

The  slide  contains  a  number  of  augites,  some  of  them  in  very  well 
defined  octagonal  cross-sections.  The  presence  of  this  mineral  associated 
in  diorites  with  hornblende  which  was  in  all  probability  dense,  is  unusual 
and  interesting  Like  the  hornblende,  the  augite  has  been  completely  con- 
verted into  chlorite,  but  the  change  from  chlorite  to  epidote  has  begun  in 
only  one  or  two  cases.  The  augite  is  sometimes  also  surrounded  with  a 
black  border.  Some  of  the  apatites  are  dark  brown  and  strongly  dichroitic. 
In  all  except  a  single  case  the  outer  edge  is  much  more  deeply  colored 
than  the  center,  but  in  one  instance  this  order  is  reversed.  Many  ordinary 
colorless  apatites  are  also  present.  The  feldspars  are  triclinic;  little  more, 
however,  can  be  said  of  them,  for  they  are  much  decomposed,  and  filled  with 
products  of  decomposition.  The  same  is  true  of  the  groundmass,  in  which 
secondary  quartz  and  calcite,  veins  and  patches  of  chlorite,  and  grains  of 
epidote  greatly  obscure  the  original  structure,  but  it  is  still  apparent  that 
it  was  granular  and  not  microlitic. 

Slide  233.    Head  of  Ophir  Eavine. 

This  slide  is  from  the  same  locality  and  the  same  cropping  as  281, 
but  from  another  specimen.  In  addition  to  the  principal  features  of  that 
slide,  it  shows  unmistakable  mica  sections,  which  have  undergone  precisely 


104  -  GEOLOGY  OF  THE  GOMSTOCK  LODE. 

the  same  changes  as  the  hornblende  and  augite  described  under  194  and 
281.  As  would  naturally  be  supposed,  the  change  to  epidote  begins  along 
cleavage  lines.     The  change  is  illustrated  in  Fig.  8,  Plate  II. 

Slides  482,  485,  486.     East-aod-west  dike,  just  south  of  Eldorado  croppings. 

Dikeofdioritc-porphyry. — At  tWs  polnt  &  dlkc  of  porphjritic  diorite  about  six 
feet  wide  cuts  the  granular  mass  of  Mount  Davidson.  Towards  the  center 
the  rock  is  fine-grained  but  evidently  crystalline,  with  small  porphyritic 
crystals  of  feldspar  and  hornblende.  For  about  an  inch  from  the  edge  the 
rock  is  very  dark  and  crypto-crystalline.  The  contact  with  the  granular 
diorite  is  an  absolutely  sharp  mathematical  line,  and  the  adhesion  is  vory 
strong.  Under  the  microscope  the  gray  including  rock  is  precisely  such 
as  is  described  under  sHde  213.  The  adjacent  dark  rock  is  manifestly 
the  same  as  421.  It  is  very  andesitic  in  appearance,  showing  a  microlitic 
groundmass,  with  excellent  flow  structure,  and  solid  brown  hornblendes  with- 
out black  borders.  It  also  contains  a  few  green  fibrous  hornblendes,  and  a 
good  deal  of  augite.  Even  within  the  limits  of  the  slide,  however,  it  is  ap- 
parent that  the  structure  of  the  groundmass  is  more  granular  as  the  distance 
from  the  contact  increases.  In  slide  486,  from  the  center  of  the  dike,  almost 
the  whole  of  the  hornblende  is  fibrous,  the  structure  is  granular,  and  the 
impression  is  simply  that  of  an  ordinary  granular  diorite  with  a  few  por- 
phyritical  crystals  of  feldspar.  But  a  few  of  the  hornblendes  are  partly 
brown  and  solid,  and  these  portions  pass  into  and  are  surrounded  by  green 
fibrous  hornblende  of  the  same  crystallographic  orientation. 


MICACEOUS   DIORITE-PORPHYRY. 

Slide  101.    1,000  feet  northeast  of  Silver  Hill  mine. 

Typical  example. — THs  is  a  gray-grccu  porphyry,  in  which  crystals  of  feld- 
spar of  a  very  uniform  size,  about  half  as  large  as  a  grain  of  wheat,  and 
smaller  crystals  of  mica  and  hornblende,  are  evenly  distributed  in  a  crypto- 
crystalline  groundmass.  Under  the  microscope  apatite,  titanic  iron,  and 
zii'con  also  make  theii*  appearance. 


DETAILED  DESCRIPTION  OP  SLIDES.  105 

The  mica  is  in  part  decomposed  into  quartz  and  epidote.  One  scale, 
18-28,  happens  to  be  so  exactly  in  the  plane  of  the  slide  as  to  show  no  trace 
of  dichroism.  This  scale  gives  an  almost  absolutely  constant  interference 
cross,  and  is  optically  negative.  It  is  therefore  biotite.  The  hornblendes, 
which  are  much  less  numerous  than  the  micas,  are  wholly  decomposed  to 
chlorite  and  epidote. 

The  large  feldspars  are  all  striated.  Several  of  them  are  cut  in  the 
zone  at  right  angles  to  ooPdb  and  show  lamellae  extinguishing  at  equal 
angles  on  each  side  of  the  twinning  plane.  These  angles  correspond  to 
labradorite.  One  feldspar,  which  shows  both  albitic  and  periclinic  twin- 
ning, gives  angles  of  extinction  which  differ  by  75°  in  two  successive 
lamellae,  but  the  angle  on  one  side  of  the  twinning  plane  is  8°  larger 
than  that  on  the  other.  The  crystal  is  cut  in  the  zone  oo  Poo  and  oo  Pdo , 
and  of  this  zone  so  little  is  known  that  the  crystal  cannot  be  pronounced 
anorthite.  One  of  the  feldspars  contains  a  fluid  inclusion  with  an  active 
bubble.  The  grains  of  feldspar  in  the  groundmass  are  not  well  preserved, 
but  almost  all  those  in  which  the  angle  of  extinction  is  determinable  trans- 
mit least  light  when  the  twinning  plane  is  parallel  to  the  plane  of  the  Nicols. 
It  seems  probable,  therefore,  that  they  are  oligoclase. 

The  groundmass  is  composed  chiefly  of  partially  decomposed  feldspar 
microlites  and  much  secondary  quartz,  with  some  calcite.  There  is  a  con- 
siderable amount  of  titanic  iron  in  characteristic  forms,  accompanied  by 
much  leucoxene.  This  decomposition  product  has  the  familiar  want  of  struc- 
ture close  to  the  undecomposed  ilmenite,  but  the  edges  of  the  patches  show 
a  granular  crystalline  arrangement,  as  if  the  smaller  particles  gradually 
united  into  comparatively  large  ones.  The  same  appearance  is  often  visible 
in  epidote.  There  are  further  many  colorless  apatites,  and  an  unusual 
quantity  of  zircons,  which  draw  attention  by  their  relief,  and  the  brilliant 
colors  which  they  exhibit  between  crossed  Nicols. 

Slide  172.    Sutro  Tunnel,  20,424  to  20,434  feet  from  entrance. 

This  is  a  mica-diorite  entirely  similar  to  slide  101,  except  that  it  con- 
tains large  quartzes,  in  which  are  sinuous  bays  of  groundmass.  These 
quartzes  contain  fluid  inclusions  with  active  bubbles. 


106  GEOLOGY  OF  THE  COMSTOOK  LODE. 


METAMORPHIC    DIORITE. 

Slide  295.    Amazon  dump. 

Typical  basaltic  variety. — This  Tock  is  of  a  Very  dark  iron-gray  color,  and  is 
full  of  bright  scaly  particles  of  bisilicates.  It  is  intensely  hard  and  tough. 
Under  the  microscope  it  is  seen  to  be  composed  chiefly  of  hornblende  and 
feldspar,  but  the  former  is  present  in  great  excess,  and  the  feldspar  is  so  full 
of  hornblendic  microlites  as  scarcely  to  be  recognizable.  Mica,  "chlorite, 
and  epidote  are  also  present  in  considerable  quantities. 

The  hornblende  is  of  two  varieties,  green  and  colorless.  The  colorless 
hornblende  is  wholly  undecomposed,  shows  capitally  marked  prismatic  and 
clinopinacoidal  cleavages.  It  absorbs  light  very  faintly,  but  polarizes  in 
brilliant  green  and  purple  colors,  like  augite.  Sections  parallel  to  the  ver- 
tical axis  show  angles  of  extinction  reaching  27°.  The  green  hornblende 
shows  an  equally  high  angle  of  extinction.  It  dichroizes  strongly  between 
a  bright,  very  slightly  brownish,  yellow  and  a  dark  grass-green.  It  is  often 
fibrous,  and  is  frequently  accompanied  by  decomposition  products.  The 
two  species  of  hornblende  stand  in  the  closest  relations  to  one  another.  In 
all  cases  the  colorless  variety  is  surrounded  by  the  green ;  in  cross-sections 
the  white  modification  appears  in  polygonal  spots  in  the  green ;  in  the  longi- 
tudinal sections  in  irregular  stripes.  Where  they  occur  together  in  this 
way  the  optical  orientation  of  the  two  is  in  all  cases  identical.  In  fact,  the 
relations  are  just  such  as  would  result  from  an  alteration  of  the  white  into 
green  hornblende,  and  taking  into  consideration  the  fact  that  the  green 
variety  alone  appears  to  suff"er  decomposition  into  any  other  mineral,  I  can- 
not avoid  the  conclusion  that  the  case  is  really  one  of  alteration.  The 
association  of  colorless  and  green  hornblende  is  illustrated  in  Figs.  11  and 
12,  Plate  II.  All  the  microlites  of  hornblende,  which  are  present  in  great 
quantities,  are  green.  These  microlites  are  so  numerous  in  the  feldspars 
that  the  striations  are  only  just  perceptible,  and  the  species  cannot  be  satis- 
factorily determined;  indeed,  hornblende  microlites  form  the  greater  part 
of  the  rock. 

A  considerable  quantity  of  fibrous  chlorite  occurs  between  the  micro- 


DETAILED  DESCRIPTION  OF  SLIDES.  107 

Htes  of  green  hornblende;  it  is  strongly  dichroitic,  and  extinguishes  light 
parallel  to  the  fibers.  There  is  also  much  epidote  present  in  compara- 
tively large  crystalline  masses.  The  dichroism,  high  colors  of  polariza- 
tion, and  the  angles  of  extinction  referred  to  the  cleavages,  leave  no  doubt 
as  to  the  mineral  species.  A  few  quartz  grains  are  scattered  through  the 
mass.  The  slide  contains  many  minute  scales  of  brown  mica,  but  no  well- 
developed  crystals.  Its  quantity  is  insignificant  as  compared  with  that  of 
the  hornblende. 

The  iron  oi'e  is  very  characteristic  ilmenite,  occurring  in  groups  of  par- 
ticles which  look  as  if  they  had  been  produced  by  chopping  a  larger  mass, 
and  is  accompanied  by  a  little  leucoxene. 

Slide  429.    3,000  feet  southeast  of  Basalt  Hill. 

Granitoid  variety. — This  is  a  piukish-gray  rock  of  granitoid  structure,  with 
many  lath-like  feldspars,  and  a  somewhat  waxy  look.  In  fact,  its  general 
appearance  resembles  that  of  many  diabases,  but  close  inspection  with  the 
unaided  eye  discloses  small  crystals  of  hornblende  and  mica.  Under  the 
microscope  quartz  grains  and  some  subsidiary  minerals  are  added  to  the  list_ 

The  hornblende  is  for  the  most  part  green  and  fibrous,  a  few  patches 
showing  a  tendency  to  brown  shades.  It  is  all  partially  decomposed,  and 
is  far  inferior  to  the  feldspar  in  quantity,  and  has  evidently  crystallized  later. 
Only  a  few  flakes  of  mica  are  visible.  The  feldspar  is  for  the  most  part 
polysynthetic,  and  the  lamellae  are  excessively  thin.  The  angles  of  extinc- 
tion of  the  sections  cvit  at  right  angles  to  the  twinning  plane  indicate  oligo- 
clase  as  the  species.  There  are  no  microlites  of  feldspar  so  developed  as  to 
justify  inferences  concerning  the  species.  A  large  part  of  the  interstices 
between  the  crystals  are  filled  with  quartz  grains,  which  are  evidently  not 
of  secondary  origin.     They  contain  exceedingl}'  minute  fluid  inclusions. 

There  is  a  large  amount  of  titanic  iron  in  this  slide,  recognizable  by 
its  cleavages  and  accompanying  leucoxene.  This  latter  mineral  is  intimately 
associated  with  sphene,  and  indeed  possibly  passes  over  into  it.  Sphene 
also  occurs  in  patches  independently  of  decomposed  ilmenite.  Though 
determinable  crystals  are  not  visible,  the  characteristically  irregular  shape- 
of  the  masses  both  as  to  outline  and  surface,  the  high  refraction,  the  feebly 


108  GEOLOGY  OF  THE  COMSTOCK  LODE, 

chromatic  tints  between  crossed  Nicols,  and  in  one  or  two  instances  the 
cleavage,  make  the  diagnosis  fairly  certain.  Besides  the  ilmenite  thei-e 
appears  to  be  a  certain  quantity  of  magnetite,  which  "is  not  improbably 
titaniferous,  for  while  the  crystal  forms  are  referable  to  the  cube  there  is  no 
accompanying  limonite.  Finally,  there  are  numerous  well-crystallized 
zircons  and  a  few  ordinary  apatites. 

Slide  293.    700  feet  southwest  of  Devil's  Gate. 

Intermediate  variety. — This  rock  is  intermediate  in  character  between  slides 
295  and  429.  It  is  crowded  with  green  hornblende  microlites,  but  not  to 
such  an  extent  as  to  conceal  the  feldspar,  which  shows  the  angles  of  extinc- 
tion proper  to  oligoclase.  It  also  contains  much  quartz  and  ilmenite,  as  well 
as  many  apatites  and  zircons. 

This  slide  is  chiefly  remarkable  for  the  presence  of  tourmaline.  It 
occurs  in  grains  and  in  imperfect  prisms.  These  extinguish  light  parallel 
to  their  principal  axis.  They  are  very  highly  dichroitic,  showing  a  clear 
brown  color  when  pai-allel  to  the  main  axis  of  the  polarizer,  and  an  almost 
absolute  black  at  right  angles  to  this  direction. 


QUAKTZ-POEPHYRY. 

Slide  354.     1,000  feet  south  of  Lawson'8  Tunnel. 

Typical  variety. — Macroscopically  this  rock  shows  a  purplish-gray  ground- 
mass  in  which  are  separated  out  porphyritically  feldspar,  mica,  and  quartz. 
Under  the  microscope  a  few  hornblendes,  apatite,  and  iron  ores  also  make 
their  appearance. 

The  feldspars,  which  in  this  slide  are  fairly  fresh,  occur  for  the  most 
part  in  irregular  grains,  rendering  it  difficult  or  impossible  in  many  cases  to 
determine  the  crystallographic  orientation.  The  larger  part  of  the  feldspars 
are  unstriated,  and  of  these  many  are  certainly  orthoclase,  as  determined 
by  the  angles  of  extinction  referred  to  the  cleavages.  I  was  unable  to  find 
.  any  unstriated  feldspars  which,  tested  in  the  same  manner,  gave  angles 
appropriate  to  either  of  the  triclinic  feldspars.     There  is  also  a  considerable 


DETAILED  DESCRIPTION  OF  SLIDES.  109 

amount  of  plagioclase  present,  which  seems  from  the  character  of  the  band- 
ing and  the  angles  of  extinction  to  be  oligoclase.  There  is  certainly  less 
plagioclase  than  unstriated  feldspar.     The  feldspars  contain  fluid  inclusions. 

The  quartz  is  present  for  the  most  part  in  macroscopical  grains,  which 
are  bounded  in  part  by  crystalline  outlines  and  in  part  by  curved  lines. 
One  large  mass  appears  to  have  been  broken,  and  a  narrow  line  of  ground- 
mass  separates  the  parted  edges.  In  many  cases  deep  sinuous  bays  of 
groundmass  penetrate  the  quartz,  and  patches  of  groundmass  are  sometimes 
surrounded  by  it.  A  considerable  number  of  inclusions  are  sparsely  scat- 
tered through  the  quartz.  Of  these  the  fluid  inclusions  are  somewhat  in 
excess  of  the  glass.  The  glass  inclusions,  which  all  show  bubbles,  are  rather 
large,  and  often  penetrate  the  slide,  so  as  to  extinguish  light  between  crossed 
Nicols.  The  glass  is  colorless  ;  its  shape  is  often  dihexahedral.  The  fluid 
inclusions  are  smaller  but  very  characteristic,  many  of  them  having  exceed- 
ingly active  bubbles.  None  of  them  appear  to  be  carbonic  acid.  Although 
there  are  comparatively  few  inclusions  in  these  quartzes,  they  are  so  dis- 
tributed that  both  kinds  are  often  in  the  field  of  a  Hartnack  No.  7  objective 
at  once. 

The  hornblendes  are  entirely  decomposed  to  chlorite.  They  have  a 
black  border,  which  does  not  appear  to  me  to  have  resulted  from  weather- 
ing of  the  hornblende  substance.     The  mica  too  is  entirely  decomposed. 

There  is  no  large  quantity  of  iron  ore,  and  its  character  is  somewhat 
indefinite.  It  is  present  in  irregular  forms,  but  there  are  no  sections  with 
the  characteristic  cleavages  of  ilmenite;  neither  is  there  any  ferric  oxide 
accompanying  the  mineral.  The  groundmass  shows  traces  of  fluidal  struct- 
ure, and  in  places  is  pseudo-spherolitic.  There  is  no  base,  but  as  the  ground- 
mass  is  impregnated  with  decomposition  products,  calcite,  quartz,  and  minute 
grains  of  epidote,  it  is  probable  that  any  glass  that  may  have  been  present 
would  be  devitrified.  There  are  a  few  poor  zircons  and  colorless  apatites 
in  the  slide.  The  rock  is  shown  as  it  appears  under  the  microscope  in 
Fig.  27,  Plate  IV. 

Slide  304.    1,000  feet  south  by  west  of  railroad  tunnel  above  Bed  Jacket. 

A  second  example. — This  Tock  is  uot  distinguishable  macroscopically  from 
that  last  described  (slide  354).    Microscopically  it  is  also  very  similar.     The 


110  GEOLOGY  OF  THE  COMSTOOK  LODE. 

relations  of  the  feldspars  are  the  same.  A  horizontal  plate  of  mica  shows 
the  interference  figure  of  biotite.  The  quartzes  contain  good-sized  inclusions 
of  glass  and  some  exceedingly  minute  ones  which  appear  to  be  liquid.  The 
groundmass  shows  a  highly  fluidal  structure.  The  effect  is  produced  by 
elongated  aggregations  of  iron  ore,  embedded  in  nearly  colorless  mateiial. 
This  colorless  substance  appears  to  be  absolutely  isotropic  in  some  places, 
in  others  it  shows  pseudo-spherolitic  structure,  but  for  the  most  part  it  exhibits 
aggregate  polarization  as  if  it  were  a  devitrified  substance.  In  many  places 
it  is  full  of  black  microlites,  which  seem  to  radiate  from  particles  of  iron 
ore. 

Slide  353.     1,700  feet  south-southeast  of  the  Amazon. 

A  third  example. — This  rock  aud  slide  are  entirely  similar  to  the  preceding; 
the  fluidal  structure  is  less  marked  than  in  304,  but  the  pseudo-spherolitic 
structure  is  well  developed.  The  feldspar  is  mostly  orthoclase,  and  the 
quartzes  with  bays  of  groundmass,  etc.,  contain  some  glass  inclusions,  and 
a  very  few  liquid  ones. 

Slide  351.     Overman  1142,  200  feet  north  of  Caledonia,  shaft. 

Specimens  tested  by  Thouiefs  method. — A  gray  rock  entirely  similar  to  those 
already  described.  Under  the  microscope  it  is  seen  to  be  somewhat  more 
altered,  the  feldspars  being  clouded  with  calcite.  Hornblende  and  mica 
occur,  and  the  groundmass  shows  the  same  fluidal  and  pseudo-spherolitic 
structure.  The  quartzes  contain  more  inclusions  both  of  fluid  and  glass  than 
those  of  the  surface  rocks.  One  of  them  is  of  a  very  unusual  character.  It 
is  a  glass  inclusion  in  a  glass  inclusion,  the  inner  one  bearing  a  bubble.  The 
inner  glass  may  differ  slightly  in  composition,  or  may  have  solidified  at  a 
different  pressure.  This  cannot  be  a  case  of  a  cut  bubble  filled  with  balsam 
and  air,  for  if  the  instrument  be  focused  on  either  surface  of  the  quartz,  the 
inclosm-e  and  bubble  are  out  of  focus.  The  inclusion  is  shown  in  Fig.  24, 
Plate  III. 

To  test  the  nature  of  the  feldspars  in  this  rock  a  fragment  was  pulver- 
ized and  separated  in  a  solution  of  mercuric  iodide  in  potassic  iodide  of  a 
specific  gravity  of  2.65.     A  large  portion  of  the  rock  rose  to  the  surface. 


DETAILED  DESCRIPTION  OF  SLIDES.  HI 

and,  on  being  mounted  in  balsam,  proved  to  be  groundmass  and  feldspar. 
Hornblende  and  mica,  most  of  the  quartz,  some  feldspars  and  decomposition- 
products  sank. 

Slide  461.    West  end  of  railroad  tunnel  above  Red  Jacket. 

Feisitic  variety. — Tliis  is  a  greenisH  gray,  fine-grained,  rhyolitic-looking  rock. 
Under  the  microscope,  too,  it  differs  in  general  appearance  from  the  ordi- 
nary quartz-porphyries  of  the  District.  In  detail,  however,  it  is  found  to 
correspond  with  tliem.  The  quartzes,  of  which  there  are  but  few,  and  those 
minute,  carry  numerous  fluid  inclusions,  many  of  them  with  active  bubbles. 
One  of  the  quartzes  also  carries  a  comparatively  large  glass  inclusion  with 
a  cut  bubble,  the  hemispherical  space  being  of  course  filled  with  balsam. 
The  groundmass  shows  traces  of  fluidal  structure,  and  is  pseudo-spherolitic. 
in  places.  The  feldspars  are  badly  clouded,  but  a  few  are  plagioelase,  and 
the  remainder  appear  to  be  orthoclase.  Hornblende,  mica,  titanite,  and 
ilmenite  are  present.  In  short,  the  rock  appears  to  be  merely  a  feisitic 
variety  of  the  ordinary  quartz-porphyry. 

Collection  of  the  Exploration  of  the  Fortieth  Parallel.     Slides  265  and  266. 

40th  Parallel  slides. — Profossor  Zirkel's  description  of  these  slides  excellently 
represents  the  phenomena,  with  one  or  two  exceptions.  While  many  of  the 
feldspars  are  clouded  with  decomposition  products,  others  are  nearly  free 
from  extraneous  matter.  Most  of  these  are  unstriated  and  appear  to  give 
the  angles  of  extinction  of  orthoclase.  The  quartzes  of  both  slides  contain 
fluid  inclusions  with  moving  bubbles,  though  they  are  neither  very  frequent 
nor  of  large  size.  In  slide  266  there  are  good  glass  inclusions  in  quartz, 
penetrating  the  section  and  remaining  dark  between  crossed  Nicols.  The 
thin  sections  and  specimens  correspond  entirely  with  those  described  in 
this  paper  as  quartz-porphyry. 

Collection  of  the  Exploration  of  the  Fortieth  Parallel.     Slide  333. 

4oth  Parallel  slide. — This  sllde  is  vcry  graphically  described  by  Professor 
Zirkel.  It  happens  to  be  a  very  small  one,  and  shows  only  two  or  three 
minute  quartzes,  in  which  I  have  detected  no  inclusions.  The  specimen 
from  which  it  was  taken,  however,  presents  quartzes  in  abundance.     The 


112  GEOLOGY  OF  THE  COMSTOCK  LODE. 

structure  and  mineralogical  composition  of  this  slide  appear  to  me  identical 
with  that  of  the  rocks  of  the  District  described  by  Professor  Zirkel  as  dacite 
and  by  me  as  quartz-porphyries.  The  properties  of  the  triclinic  and  ortho- 
tomic  feldspars  are  the  same,  the  hornblende  and  mica  are  of  the  same  char- 
acter and  of  the  same  degree  of  decomposition,  and  the  groundmass  is 
indistinguishable.  Professor  Zirkel  draws  special  attention  to  the  fluid 
inclusions  in  the  feldspars  of  this  slide. 


EARLIER   DIABASE. 
Slide  349.     Sutro  Tunnel,  north  branch,  50  feet  south  of  Ophir. 

Typical  example. — This  is  a  gray  rock,  which  might  readily  be  mistaken  at 
first  glance  for  a  diorite.  On  close  inspection,  however,  a  certain  waxy 
luster,  characteristic  of  augitic  rocks,  is  perceptible,  as  well  as  numerous 
lath-like  feldspars  from  !""•  to  2"""-  long.  Under  the  microscope  it  is  plain 
that  the  rock  consists  of  triclinic  feldspar,  augite,  and  an  iron  ore. 

The  larger  feldspars  are  well  developed;  the  smaller  ones  are  granitoid 
in  structure,  and  appear  to  have  occupied  the  interstices  between  the  larger 
crystals.  The  larger  feldspars  show  polysynthetic  twinning,  according  to 
the  albite  law,  the  lamellae  being  of  moderate  thickness.  In  addition,  many 
of  the  individuals  show  pericline  twinning,  and  in  some  cases  polysynthetic 
individuals  are  united  as  Carlsbad  twins.  The  angles  of  extinction  are  all 
within  the  limits  appropriate  to  labradorite,  and  some  of  the  macropinacoidal 
sections  recognizable  by  the  shape,  and  by  the  angles  of  the  two  species  of 
twin  lamellae,  give  almost  exactly  the  theoretical  maximum  angle  of  extinc- 
tion on  each  side  of  the  twinning  plane.  Very  few  of  the  small  feldspars 
forming  a  sort  of  groundmass  show  crystalline  outlines ;  but  almost  all  are 
twinned,  and  many  of  them  give  angles  of  extinction  indicating  labradorite. 
In  fact,  I  was  unable  to  find  any  evidence  of  the  existence  of  any  other 
feldspar.     There  are  a  few  fluid  inclusions  in  the  feldspars. 

A  considerable  portion  of  the  augite  is  fresh.  It  i&  of  the  ordinary  pale 
brownish-yellow,  only  just  perceptibly  dichroitic,  and  in  general  exhibits 
excellent  cleavages.     Some  well-defined  octagonal  cross-sections  show  not 


DET^ULED  DESOEIPTION  OP  SLIDES.  113 

only  the  prismatic  cleavages,  but  both  the  pinacoidal  ones.  A  large  part 
of  the  augites  are  twinned,  and  many  of  them  show  polysynthetic  structure. 
In  one  case  in  another  slide,  from  the  same  region,  I  counted  thirteen  1am- 
ellse.  In  many  cases,  as  is  so  frequent  in  feldspars,  the  lamellae  do  not 
extend  entirely  through  the  crystal.  An  excellent  instance  is  represented 
in  Plate  III.,  Fig.  15.  In  this  slide  (and  many  similar  cases  have  been  found 
in  others  from  the  Sutro  Tunnel)  there  occurs  a  long,  somewhat  ill-defined 
section  of  augite,  showing  a  single  cleavage  parallel  to  the  longer  axis  and 
extinguishing  at  an  angle  of  38°,  yet  showing  planes  of  twinning  which 
cut  the  direction  of  cleavage  at  an  angle  of  32°.  At  first  sight  this  gives 
the  impression  of  a  pinacoidal  section,  and  a  twin  with  a  hitherto  unob- 
served face  of  composition.  In  reality  it  is  a  section  at  a  considerable  angle 
to  the  principal  axis,  and  cutting  a  prismatic  face  nearly  parallel  to  the 
edge  0  P,  00  P.  The  second  system  of  cleavages  does  not  appear  in  this 
instance,  because  it  cuts  the  section  at  a  very  low  angle.  Such  sections 
must  occur  in  all  augite  rocks,  but  attract  attention  here  on  account  of  the 
prominence  of  the  twinning.^ 

A  portion  of  the  augite  is  converted  into  uralite.  This  product  is 
strongly  dichroitic,  light  greenish-yellow  in  color,  and  of  course  fibrous  in 
texture.  The  crystallographic  orientation  is  often  the  same  over  considerable 
areas,  and  these  show  the  angles  of  extinction  characteristic  of  hornblende. 
In  some  cross-sections,  too,  an  excessively  fine  cleavage  at  an  angle  of 
about  125°  can  be  made  out  with  high  powers.  The  conversion  into  uralite 
seems  to  have  proceeded  with  little  regularity,  sometimes  attacking  the 
augite  from  the  outside,  and  sometimes  along  cleavages  and  fractures.  The 
direction  of  the  fibers  of  uralite  is  not  in  general  that  of  the  augite  cleavage, 
but  usually  not  very  far  from  it. 

The  uralite  is  further  often  converted  into  chlorite  of  a  darker  green 
color  and  equal  dichroism.     The  fibers  of  this  product  extinguish  parallel 

'When  there  is  reason  to  suppose  that  a  section  showing  an  oblique  trace  of  a  twinning  plane 
cuts  one  of  the  prism  faces  lying  next  to  the  clinopinacoid  parallel  to  the  edge  between  this  face  and 
the  base,  the  approximate  position  of  the  section  can  readily  be  inferred ;  for  if  the  prism  angle  were 
90°,  the  tangent  of  the  angle  at  which  the  trace  of  the  twinning  j)]ane  cuts  the  longitudinal  striations, 
would  be  equal  to  the  sine  of  the  angle  at  which  the  section  cuts  the  main  axis  of  the  crystal.  As  the 
angle  of  ex  P  is  only  87J  degrees,  the  observed  and  the  calculated  angle  will  be  too  large,  but  the  error 
will  reach  a  maximum  of  2^  degrees  only  in  sections  at  right  angles  to  the  main  axis. 
8  C  L 


114  GEOLOGY  OF  THE  COMSTOOK  LODE. 

to  their  direction,  and  polarize  for  the  most  part  in  dark  bluish  tints.  In 
many  cases  the  uralite  seems  to  be  attacked  from  innumerable  points,  and 
the  chlorite  then  shows  a  spherolitic  sti'ucture.  There  are  a  few  grains  of 
epidote  in  this  slide,  associated  in  a  somewhat  indefinite  manner  with  the 
uralite  and  the  chlorite. 

The  iron  ore  seems  to  be  ilmenite.  It  occurs  in  the  characteristic  forms 
of  that  mineral,  and  is  accompanied  by  a  very  little  leucoxene.  There  are 
also  a  very  few  apatites,  a  little  quartz,  which  is  probably  secondary,  and 
one  or  two  particles  of  sphene. 

Slide  18.     Sutro  Tunnel.    Hanging  wall  of  Lode  at  Savage  connection. 

Fresh  diabase  used  for  experiments. — This  iu  hand  spccimcns  is  a  vcry  black  rock, 
with  less  waxy  luster  than  most  diabases  show,  but  with  the  usual  lath-like 
feldspars.  Tlie  feldspar  does  not  differ  from  that  in  slide  349,  and  measure- 
ments of  the  angles  of  extinction  show  it  to  be  labradorite.  It  contains 
some  fluid  inclusions.  Most  of  the  augite  is  fresh,  and  some  crystals  show 
zonal  structure ;  a  few  are  converted  into  uralite  and  chlorite.  The  ground- 
mass  of  the  rock  contains  many  microlites  of  augite.  There  are  a  few 
flakes  of  a  brown,  highly  dichroitic  mineral  in  this  slide,  which  show  none 
of  the  structure  of  hornblende,  and  seem  to  be  biotite.  Its  quantity  is 
insignificant.     The  iron  ore  is  at  least  in  part  ilmenite. 

This  is  the  freshest  diabase  known  to  exist  in  the  District,  and  as  such 
was  selected  for  the  experiments  on  kaolinization.  Assays  and  a  chemical 
analysis  of  it  will  be  given  at  the  end  of  the  chapter.  Its  appearance  under 
the  microscope  is  illustrated  in  Plate  IV.,  Fig.  28. 

Slide  53.     Sutro  Tunnel,  19,200  feet  from  entrance. 

Quartzose  diabase. — Macroscoplcally  thls  rock  entirely  resembles  that  repre- 
sented by  slide  18.  The  slide  is  one  of  the  few  containing  quartzes  which 
are  unquestionably  primitive.  In  this  case  the  arrangement  of  the  microlites 
of  iron  ore  round  their  edges,  and  the  inclusions  of  groundmass,  put  their 
character  beyond  question.  These  quartzes  are  remarkably  full  of  fluid 
inclusions;  the  smaller  ones  with  spontaneous  bubbles,  which  do  not  decrease 
in  size  when  the  sHde  is  heated  to  above  40°  C.  The  rock  contains  com- 
paratively little  fresh  augite. 


DETAILED  DESCRIPTIO]Sr  OF  SLIDES.  115 

Slide  346.     Siitro  Tunnel,  south  branch,  3,960  feet  from  fork. 

Hornbiendic  diabase. — Macroscopicallj  this  Tock  looks  much  like  those  already 
described,  except  that  it  contains  a  considerable  number  of  clearly  recog- 
nizable hornblendes.  Three  or  four  of  these  occur  in  the  thin  section. 
They  are  bright  brown  in  color,  and  decomposition  has  scarcely  set  in. 
Far  more  numerous  are  the  augite  sections,  which,  though  wholly  decom- 
posed to  uralite  and  chlorite,  retain  their  characteristic  outline.  In  some  of 
these  the  conversion  of  chlorite  into  epidote  may  be  traced.  The  relations 
of  the  porphyritical  crystals  to  the  groundmass  in  this  slide  are  precisely 
those  met  with  in  the  ordinary  diabases  of  the  District. 

Slide  396.     Yellow  Jacket  shaft,  2,299  feet  from  surface. 

Diabase  containing  epidote. — Thls  Is  a  greonisli-gray  granular  rock,  somewhat 
unusual  in  color  for  Washoe  diabase.  In  most  cases  in  this  District  grains 
of  epidote  may  be  observed  under  the  microscope,  developed  in  the  chlorite 
formed  by  the  decomposition  of  the  augite;  but  this  change  seems  to  cease 
almost  as  soon  as  begun.  In  the  more  decomposed  rocks  the  chlorite  is 
seen  passing  into  calcite  and  quartz,  while  the  epidote  grains  are  replaced 
by  an  opaque  substance,  which  is  probably  iron  oxide.  In  this  slide,  how- 
ever, it  is  plain  that  augite  has  passed  into  uralite,  this  into  chlorite,  and 
that  a  great  part  of  the  chlorite  has  been  converted  into  epidote.  All  the 
stages  can  be  observed  here,  as  in  the  McKibhen  Tunnel  diorite,  and,  as  in 
that  rock,  the  crystals  of  epidote  are  seen  eating  their  way  into  the  chlorite. 

Slide  134.     Sierra  Nevada,  1,450,  north  drift,  217  feet  north  of  shaft. 

Diabase  containing  diaiiage. — Macroscoplcally  this  Is  a  dark,  fine-grained  rock, 
which  looks  more  hke  some  of  the  dark  diorites  than  it  does  like  diabase. 
Under  the  microscope  it  is  seen  to  be  composed  of  rather  uniform  grains  of 
plagioclase,  diaiiage,  and  hornblende. 

The  plagioclase  is  broad-banded,  contains  fluid  inclusions,  and  gives 
the  angles  of  extinction  of  labradorite.  The  hornblende  is  bright  brown. 
The  diaiiage,  which  is  much  in  excess  of  the  hornblende,  is  dark  gray  and 
feebly  diaphanous.  In  general  it  is  disposed  in  irregular  patches  between 
the  feldspars,  but  there  are  a  few  sections  with  the  augite  outline,  and 


116  GEOLOGY  OF  THE  OOMSTOGK  LODE. 

showing  close  partings  in  a  pinacoidal  direction.  It  transmits  light  too 
feebly  to  permit  of  exact  determinations  of  angles  of  extinction,  but  angles 
of  about  30°  were  noted. 

The  occurrence  of  the  rock  is  purely  local,  and  I  regard  it  as  a  mere 
modification  of  the  diabasitic  rocks,  and  as  not  sufficiently  independent  to 
be  classified  as  gabbro. 


YOUNGER   DIABASE. 

Slide  466.     Ghollar,  1,900  foot  level;  40  feet  east  of  incline. 

The  only  variety. — Thls  is  9,  bluish-black  finc-graincd  rock,  without  a  trace 
of  porphyritic  structure.  Under  the  microscope  it  seems  to  be  composed  of 
plagioclase,  augite,  and  magnetite.  The  feldspars  present  lath-like  forms 
of  nearly  equal  size ;  they  give  angles  of  extinction  corresponding  to  labra- 
dorite,  and  show  no  distinguishable  inclusions  besides  augite  microlites.  The 
augite  is  mostly  granular,  and  with  the  magnetite  fills  the  interstices  between 
the  feldspars.     It  is  somewhat  dichroitic. 

The  slide  is  considerably  obscured  by  clouds  of  a  smoky  brownish  sub- 
stance, which  possesses  no  visible  structure  and  no  dichroism,  but  shows 
aggregate  polarization.  It  is  the  formation  of  this  substance  which  turns 
fresh  fractures  of  the  black  dike  from  the  bluish  color  known  by  draughts- 
men as  "neutral  tint"  to  a  smoky  brown  after  a  few  hours'  exposure.  There 
is  but  one  variety  of  the  black  dike,  and  it  is  almost  impossible  to  distinguish 
slides  of  this  rock  from  different  parts  of  the  Lode.  A  characteristic  field 
of  this  slide  is  shown  in  Fig.  29,  Plate  V.  A  specimen  of  the  diabase  from 
Orange,  N.  J.,  showed  a  tendency  to  the  same  alteration  in  color  after  a  few 
days'  exposure,  and  a  slide  from  it  exhibits  the  same  peculiarities. 


EAKLIEE    HORNBLENDE-ANDESITES. 

Slide  309.    Edge  of  plateau,  northwest  of  Ophir  Hill. 

Typical  rock. — This  is  a  porphyritlc  rock,  in  which  crystals  of  feldspar 
and  hornblende  are  separated  out  in  a  bluish-gray  groundmass.      Under 


DETAILED  DESCRIPTION  OF  SLIDES.  117 

the  microscope  a  large  amount  of  augite  and  some  apatite  and  iron  ore  make 
their  appearance. 

The  feldspars  aj^pear  to  be,  without  exception,  triclinic.  The  large 
crystals  give  labradorite  angles,  while  the  microlites  appear  to  be  for  the 
most  part  referable  to  oligoclase.  The  large  feldspars  in  these  andesites 
very  commonly  show  both  albite  and  periclinic  twinnings,  and  polysyn- 
thetic  individuals  are  frequently  combined  as  Carlsbad  twins.  The  feldspars, 
which  strangely  enough,  considering  the  fresh  condition  of  the  bisilicates, 
are  largely  converted  into  calcite  and  quartz,  contain  some  glass  inclusions. 

Hornblende  is  present  only  in  large  masses  of  somewhat  irregular  out- 
line, surrounded  by  a  deep  black  border.  The  substance  of  the  hornblende 
is  for  the  most  part  quite  fresh,  and  of  a  deep  greenish-brown.  It  contains 
minute  opaque  inclosures,  which  are  probably  of  the  same  nature  as  those 
described  under  slide  421.  The  augites  are  very  numerous,  but  small. 
The  percentage  of  the  two  silicates  cannot  differ  greatly,  but  the  hornblendes 
give  the  character  to  the  rock.  The  augites  are  very  perceptibly  dichroitic, 
and  are  often  crystallographically  well  developed.  Many  of  them  are  twinned 
and  some  are  decomposed  to  fibrous  chlorite,  which  polarizes  in  dark  bluish 
colors.  There  is  much  of  this  mineral  in  the  slide  which  has  evidently 
been  transported,  and  has  settled  in  patches  in  which  there  is  a  strong  tend- 
ency to  spherolitic  arrangement.  The  patches  of  chlorite  are  accompanied 
by  quartz,  which  usually  occupies  the  periphery.  In  some,  cases  particles 
of  epidote  may  be  seen  in  the  chlorite. 

The  groundmass  is  made  up  of  microlites  of  oligoclase,  with  a  con- 
siderable amount  of  augite,  magnetite,  and  apatite.  The  last  is  almost  all 
of  a  deep  brown  color,  and  in  consequence  markedly  dichroitic.  There  is 
scarcely  a  trace  of  fluidal  structure  in  this  slide. 

Slide  228.    Knoll  northwest  of  Combination  shaft. 

A  second  typical  specimen. — This  Is  a  purpHsh-gray  rock,  and  in  that  respect 
peculiar.  Under  the  microscope  it  is  very  similar  to  that  last  described. 
It  shows  a  decided  fluidal  structure,  but  no  glass  base.  The  feldspars  con- 
tain good  glass  inclusions.  Some  of  the  hornblendes  are  twinned.  In  spite 
of  its  purplish  color  this  hornblende-andesite  is  microscopically  typical  of 
the  Washoe  occurrences,  and  is  illustrated  in  Fig.  30,  Plate  V. 


118  GEOLOGY  OF  THE  COMSTOOK  LODE, 

Slide  229.    From  the  same  locality  as  228. 

Specimen  containing  iimenite. — Tliis  coiitains  ail  augltc  wliich  shows  excelleiit 
pinacoidal  as  well  as  prismatic  cleavage.  It  also  contains  a  few  patches 
of  a  finely  granular  mineral,  whicli  shows  very  feeble  tints  between  crossed 
Nicols,  and  might  be  taken  for  sphene.  It  is  in  reality  epidote,  which  often 
behaves  in  this  way  when  finely  divided. 

The  hornblendes  in  this  slide  are,  as  a  rule,  less  decomposed  than  the 
augites.  Indeed,  the  hornblendes  in  the  Washoe  andesites  frequently, 
though  by  no  means  always,  resist  decomposition  better  than  the  augites, 
perhaps  on  account  of  the  heavy  black  border.  Much  of  the  chlorite  formed 
from  the  augite  has  further  decomposed  into  calcite  and  quartz.  One 
pseudomorph  of  chlorite  after  augite  has  been  attacked  from  within  by  epi- 
dote,  and  from  without  by  calcite. 

To  test  the  nature  of  the  iron  ore  in  this  rock  the  cover  of  the  slide 
was  removed,  and  the  balsam  well  washed  off  with  alcohol.  Careful  draw- 
ings were  made  with  the  camera  of  certain  portions  of  the  slide,  which  were 
then  treated  with  strong  chlorhydric  acid.  A  drop  of  acid  was  placed  upon 
the  area  to  be  tested  and  the  slide  warmed  over  a  lamp  for  several  minutes. 
The  acid  was  then  washed  off  with  water,  and  the  operation  repeated  five 
times.  After  each  treatment  the  slide  was  inspected,  and  the  result  showed 
that  while  the  black  border  and  certain  grains  of  iron  ore  were  completely 
soluble,  others  were  only  coated  with  a  white  film,  and  remained  undis- 
solved. The  etching  also  brought  out  faint  straight  lines  on  the  undissolved 
grains,  at  an  angle  of  approximately  60°,  which  seems  to  complete  the 
proof  that  the  mineral  is  iimenite.  I  find  myself  unable  to  distinguish 
under  the  microscope  the  difference  in  tint  between  magnetic  and  titanic 
iron,  which  is  so  perceptible  in  the  streak. 

Slide  208."    North  Twin  Peak. 

Partially  decomposed  hornbiende-andesite. — A  dark,bluish,  fresh-lookingandesitc,  but 
in  reality  much  more  decomposed  than  those  just  described.  The  feldspars 
contain  glass  inclusions,  but  no  fluid  ones  were  observed.  They  are  but 
slightly  decomposed,  showing  a  little  calcite  and  a  few  porous  streaks  and 
spots.    They  contain  many  yellow,  rounded  microlites,  some  of  which  extin- 


DETAILED  DESCRIPTIOlSr  OF  SLIDES.  119 

guish  light  at  an  angle  of  above  30°,  and  are  probably  augite.  The  rest 
of  the  augite  is  decomposed  to  chlorite,  of  which  there  are  excellent  pseu- 
domorphs.  The  hoi'nblende,  too,  is  decomposed.  With  a  low  power  it 
seems  as  if  the  space  within  the  heavy  black  border  were  filled  with  calcite, 
quartz,  and  magnetite ;  but  a  No.  7  objective  shows  that  the  apparently 
opaque  paii;icles  are  in  reality  minute  grains  of  a  strongly  refracting  min- 
eral, no  doubt  epidote.  Epidote  in  determinable  grains  also  occurs  in  the 
chlorite  masses.  There  is  considerable  magnetite  in  this  slide,  as  well  as 
many  colorless  apatites  and  one  or  two  zircons.  The  groundmass  shows 
well  marked  fluidal  structure,  but  no  glass  base. 

This  is  the  rock  described  by  Professor  ZIrkel  as  from  the  first  hill 
north  of  Gold  Hill  Peak,  and  analyzed  by  Dr.  Kormann. 

Slide  209.    Quarry  1,000  feet  west  of  Yellow  Jacket  east  shaft. 

Considerably  decomposed  hornblende-andesite. A  gray-greCU  pOrphyritic  TOCk,  imme- 
diately overlying  and  passing  into  ordinary  bluish  hornblende-andesite. 
Under  the  microscope  it  appears  that  the  bisilicates  are  wholly  decomposed, 
the  hornblendes  being  traceable  only  by  the  black  borders  now  filled  with 
quartz,  calcite,  and  oxides.  A  few  pseudomorphs  of  chlorite  after  augite 
remain.  The  feldspars  also  are  considerably  attacked,  and  contain  second- 
ary fluid  inclusions. 

Slide  210.    500  feet  north  of  North  Twin  Peak. 

Much  decomposed  specimen. — Thls  specimeu  is  ft'om  tlic  samc  mass  of  rock 
as  that  represented  by  slide  209,  and  resembles  it,  except  in  the  fact  that  the 
feldspars  have  lost  their  transparency.  Under  the  microscope  it  is  also 
plain  that  it  is  the  same  rock  in  a  more  advanced  stage  of  decomposition. 
The  feldspars  are  in  part  filled  with  specks  of  calcite ;  in  part  the  calcite 
appears  to  have  been  removed  by  solution,  and  in  some  instances  the  cavi- 
ties thus  formed  seem  to  have  been  filled  with  liquid,  accompanied  by  a 
bubble;  or  in  other  words  the  feldspars  contain  secondary  liquid  inclusions. 

Secondary  fluid  inclusions. — I  basc  tlic  opiuiou  that  thcsc  inclusions  are  second- 
ary on  the  following  grounds:  They  do  not  occur  in  the  fresh  hornblende- 
.  andesite  from  the  same  locality,  or  in  unattacked  feldspars  in  decomposed 


120  GEOLOGY  OF  THE  COMSTOCK  LODE. 

andesites.  They  are  accompanied  by  particles  of  calcite,  and  by  cavities 
which  entirely  resemble  them  in  outline  and  general  character.  While 
primitive  fluid  inclusions  are  either  negative  crystals,  or  more  or  less  dis- 
torted vesicles,  and  are  bounded  by  smooth  curves  of  greater  or  less  com- 
plexity, these  inclusions,  as  a  rule,  show  irregular  edges  composed  of  broken 
lines.  It  is  of  course  necessary  that  these  inclusions  should  at  some  time 
have  had  a  connection  with  the  minute  water  channels  of  the  rock  mass 
through  capillary  fissures,  but  it  by  no  means  follows  that  these  would 
appear  even  under  high  powers  if  open,  and  nothing  is  more  probable  than 
that  they  should  often  be  closed  by  decomposition  products.  I  have  but 
rarely  observed  an  active  bubble  in  inclusions  of  this  class. 

Similar  inclusions  have  been  observed  in  the  decomposed  andesites 
of  other  localities  in  the  District,  and  in  the  same  relations  to  the  decompo- 
sition of  the  feldspars.  While  in  typical  instances  it  appears  to  me  easy  to 
discriminate  between  primary  and  secondary  liquid  inclusions  in  feldspars, 
cases  may  arise  in  which  a  confusion  is  possible.  There  is  no  reason  why 
such  inclusions  should  not  occur  in  the  older  rocks  as  well  as  in  the  andes- 
ites, and  indeed  they  appear  to  me  to  do  so,  especially  in  the  quartz-por- 
phyries. I  have  not  alluded  to  them  in  describing  slides  of  the  older  rocks, 
because  they  are  there  accompanied  by  primitive  inclusions,  and  it  seemed 
best  to  mention  the  subject  in  connection  with  a  rock  in  which  primitive 
fluid  inclusions  are  very  exceptional.  I  have  borne  the  matter  in  mind, 
however,  and  have  not  used  the  presence  of  fluid  inclusions  as  a  diagnostic 
point,  except  where  their  primitive  character  appeared  certain.  A  secondary 
inclusion  from  slide  210  is  shown  in  Fig.  22,  Plate  III.^ 

Slide  311.    1,200  feet  northwest  of  Geiger  Grade  Toll  House. 

Specimen  showing  disseminating  hornblende. This    is    a    light    blulsh-gray,  Ordluary- 

looking  andesite,  with  rather  a  large  number  of  visible  hornblendes.  Under 
the  microscope  it  is  remarkable  from  the  fact  that,  besides  the  hornblendes 
which  are  apparent  to  the  unaided  eye,  it  contains  a  vast  number  of  spiculse 
of  the  same  mineral  disseminated  through  the  groundmass.     In  the  thin  sec- 

'  Mr.  C.  W.  Cross,  In  his  Studien  tiber  Bretonische  Gesteine,  Vienna,  Holder,  has  called  attention 
to  secondary  fluid  inclusions  of  a  different  origin  and  character. 


DETAILED  DESCRIPTIOIT  OP  SLIDES.  121 

tion  these  hornblendes  are  of  a  light  yellowish-brown  color,  and  are  not  ac- 
companied by  black  borders.  Very  many  of  the  hornblendes  are  twinned, 
and  a  few  show  zonal  structure.  The  hornblende  is  strongly  dichroitic  and 
gives  angles  of  extinction  reaching  20°.  The  augites  are  few  in  number 
and  minute;  indeed,  at  first  sight,  there  seems  to  be  no  augite  at  all. 

The  feldspars  are  all  triclinic,  but  are  considerably  decomposed,  and  it 
is  not  easy  to  determine  their  angles  of  extinction  with  accuracy.  Most  of 
the  large  crystals,  however,  give  angles  which  fall  within  the  limits  of  lab- 
radorite,  while  the  microlites  seem  to  be  oligoclase;  but  one  crystal  show- 
ing the  two  ordinary  striations  gives  angles  of  almost  exactly  37°.  This 
then  must  be  anorthite.  It  is  impossible  to  say  that  the  other  large  crystals 
are  not  so,  but  the  probabilities  are  that  others  would  have  been  found 
exceeding  the  limits  of  labradorite  had  such  been  the  case.  In  one  of  the 
large  feldspars  fluid  inclusions  of  the  kind  called  secondary  were  observed, 
and  one  of  these  contained  a  slowly  moving  bubble.  Some  of  the  feldspars 
also  contain  partially  devitrified  glass  inclusions. 

The  slide  shows  two  or  three  small  grains  of  quartz  which,  from  the 
arrangement  of  the  particles  of  the  surrounding  groundmass,  appear  to  be 
primary.  They  contain  liquid  inclusions  with  moving  bubbles.  This  is 
the  only  case  in  which  primitive  fluid  inclusions  have  been  detected  in  the 
Washoe  andesites.  The  opacite  is,  for  the  most  part  at  all  events,  magnetite. 
A  very  perfect  hexagonal  crystal  may  be  a  section  of  a  dodecahedron. 
The  apatite  is  colorless  and  without  peculiarities.  The  groundmass  polarizes 
throughout,  though  in  places  only  very  feebly.  If  it  ever  contained  any 
base,  the  glass  is  now  nearly  or  quite  devitrified. 

Slide  464.    1,200  feet  northwest  of  Geiger  Grade  ToU  House. 

Coarse-grained  trachytic-looking  hornblende-andesite. This  slidc  Is  frOm  the  SamO  Crop- 
ping as  311,  and  beyond  the  possibility  of  a  doubt  the  same  rock,  but  it 
diff"ers  greatly  in  appearance,  being  coarse-grained,  gray,  and  more  like  an 
ordinary  trachyte  than  a  common  andesite  in  habitus.  Under  the  mi- 
croscope it  is  manifestly  the  same  rock,  though  with  a  modification  of 
structure,  for  the  groundmass  is  granular  instead  of  microlitic.  There  are  a 
few  grains  of   quartz  which  carry  fluid  inclusions.      Slide  311   contains 


122  GEOLOGY  OF  THE  COMSTOCK  LODE. 

remarkably  few  augites,  while  in  this  I  was  unable  to  find  a  single  one,  in 
which  respect  it  and  slide  375  form  the  only  exceptions  among  the  earlier 
andesites  of  the  District.  The  hornblende  is  of  the  same  color  and  general 
character  as  that  in  slide  311,  but  the  crystals  are  fewer  in  number  and 
larger  in  size.  The  decomposition  of  the  hornblende  in  this  specimen  is 
peculiar  and  interesting.  The  first  change  appears  to  have  been  to  chlorite 
masses,  of  which  a  few  are  still  surrounded  by  fresh  hornblende.  Some  spots 
of  this  chlorite  contain  bunches  of  epidote,  evidently  formed  from  it,  but 
much  of  the  chlorite  has  been  converted  into,  or  has  given  place  to,  quartz. 
Decomposition  has  set  in  along  cracks  or  cleavages  of  the  hornblende  crys- 
tals, producing  little  veins  of  chlorite,  and  the  substitution  of  quartz  for 
chlorite  has  subsequently  taken  place  from  the  hornblende  walls  of  the  veinlet 
towards  the  central  line,  but  has  sometimes  left  a  narrow  seam  of  chlorite 
along  the  middle  of  the  vein.  This  is  shown  in  Fig.  3,  Plate  II.  A  question 
might  be  raised  as  to  whether  the  quartz  had  not  first  partially  filled  the  veins, 
the  chlorite  representing  a  subsequent  infiltration ;  but  the  thoroughly  fresh 
condition  of  the  hornblende  walls  seems  to  forbid  such  a  supposition.  In  some 
of  the  smaller  crystals  a  fresh  kernel  of  hornblende  is  seen  surrounded  by 
a  zone  of  quartz,  and  this  again  by  a  narrow  border  of  epidote.  Taking  the 
appearance  just  described  into  account,  it  appears  to  me  probable  that  these 
hornblendes  were  in  process  of  conversion  into  chlorite  from  the  edges,  and 
that  an  alteration  to  epidote  had  begun  on  the  periphery,  when  the  silicify- 
ing  action  set  in,  leaving  the  hornblende  and  the  epidote  unaffected.  The 
hornblendes  carry  small  bubble-bearing  glass  inclusions.  The  slide  also 
contains  much  decomposed  mica.  That  mineral  has  been  replaced  by  chlo- 
rite and  this,  again,  is  full  of  patches  of  epidote,  evidently  parasitic  on  the 
chlorite.  A  portion  of  this  chlorite,  as  well  as  that  derived  from  horn- 
blende, appears  to  have  been  converted  into  quartz.  The  feldspars  are 
much  decomposed,  but  are  evidently  triclinic. 

Slide  454.    Cedar  Hill  Caiioa,  1,500  feet  due  west  of  Water  Tunnel. 

Highly  augitic  variety. — This  is  a  dark  bluish  rock,  which  shows  a  considerable 
number  of  macroscopical  hornblendes.  Under  the  microscope  the  augite  is 
seen  to  predominate  over  the  hornblende,  but  as  it  occurs  in  a  typical  horn- 


DETAILED  DESCEIPTIOlSr  OF  SLIDES.  123 

blende-andesite  area,  has  a  microHtic  groundmass,  and  shows  considerable 
hornblende,  I  have  regarded  the  excess  of  augite  as  local.  The  slide  is 
remarkable  for  the  fact  that  much  of  the  strongly  dichroitic  augite  is  sur- 
rounded by  a  black  border  quite  as  broad  as  that  which  ordinarily  occurs 
about  andesitic  hornblendes,  though  not  so  broad  as  that  accompanying  the 
hornblendes  in  this  specimen.  This  slide  contains  unquestionable  ilmenite 
with  rhomboidal  cleavage  marked  by  translucent  lines.  One  of  these  is 
shown  in  Fig.  19,  Plate  III.  O-ne  of  the  masses  of  ilmenite  incloses  a  twin 
augite  crystal,  just  as  the  same  mineral  so  commonly  includes  apatite.  The 
apatites  are  mostly  deep  brown  and  dusty. 

Slide  450.     1,000  feet  east  of  station  at  jimction  of  Silver  City  Eailroad. 

Specimen  showing  hornblende  with  double  black  border. Thls     rOck    Is     Ouly    OXpOSed    by 

the  railroad  cut  for  a  few  yards,  and  undoubtedly  underlies  the  adjoining 
augite-andesite.  It  is  dark  purplish  gray,  and  contains  a  very  large  amount 
of  visible  hornblende.  The  slide  is  chiefly  remarkable  for  the  light  which 
it  throws  on  the  character  of  the  black  border.  The  hornblendes  are  devel- 
oped with  unusual  sj^mmetry,  but  many  of  the  crystals  have  been  broken,  and 
all  the  fragments  are  surrounded  by  black  borders.  In  one  case  a  beau- 
tifully fresh,  highly  dichroitic,  dark  brown  hornblende  fragment  shows  not 
only  a  black  border  but  a  parallel  band  of  magnetite  at  some  distance  from 
the  edge — a  zonal  structure  marked  by  an  interior  black  belt.  This  crystal 
is  shown  in  Fig.  17,  Plate  III.  Other  of  the  large  hornblendes  in  the  slide 
show  the  same  phenomenon,  though  imperfectly;  but  the  small  crystals 
have  but  a  single  border.^ 

The  specimen  contains  considerable  augite,  arid  the  groundmass  shows 
fluidal  structure,  as  well  as  the  peculiar  felt-like  texture  so  common  in  augite- 
andesites.  It  is  possibly  not  a  hornblende-andesite,  in  spite  of  the  great 
predominance  of  hornblende,  but  an  augite-andesite  with  a  local  segre- 
gation of  hornblende.  No  other  hornblende-andesite  occurs  for  a  long 
distance,  and  a  glance  at  the  map  will  show  the  improbability  of  any  con- 
siderable amount  of  that  rock  being  entirely  covered  by  the  limited  areas 
of  augite-andesite. 

'For  some  speculations  on  this  occurrence  see  page  59. 


124  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Slide  375.     Outcrop  at  junction  of  Sutro  and  Quarry  roads. 

Micaceous  hornbiende-andesite. — This  is  a  somewhat  trachytic-looking  rock,  with 
very  white  feldspars  embedded  in  a  rough  gray  groundmass.  Mica  is  also 
visible,  though  not  prominent.  Under  the  microscope  it  is  plainly  only  a 
micaceous  variety  of  the  surrounding  hornbiende-andesite.  The  feldspar  is 
wholly  triclinic,  and  the  large  crystals  give  the  angles  of  extinction  of 
labradorite.  They  are  much  decomposed,  but  contain  recognizable  glass 
inclusions.  There  are  also  numerous  secondary  fluid  inclusions.  The  mica 
is  decomposed,  largely  to  chlorite  and  epidote.  There  are  also  hornblendes, 
or  rather  their  outlines.  Much  of  the  groundmass  is  devoid  of  microlites, 
shows  a  feeble  aggregate  polarization,  and  is  probably  a  partially  devitri- 
fied  glass.     I  could  find  nothing  which  could  be  interpreted  as  augite. 

Slide  326.     Sutro  Tunnel,  17,100  feet  from  entrance. 

Specimen  showing  stages  of  decomposition. — This  is  a  grceuish-gray  rock,  with  por- 
phyritical  crystals  of  feldspar  and  hornblende.  It  also  shows  some  pyrites, 
and  has  evidently  undergone  considerable  decomposition.  Under  the  micro- 
scope the  slide  shows  much  brown  hornblende,  some  augite,  triclinic  feld- 
spars, and  an  andesitic  groundmass.  The  hornblendes  are  peculiarly  inter- 
esting because  they  exhibit  the  process  of  decomposition  in  all  its  stages. 
The  hornblende  is  brown,  much  of  it  is  twinned,  and  none  of  it  shows  black 
borders.  The  first  step  in  the  degeneration  is  the  formation  of  chlorite,  which, 
of  course,  largely  follows  the  cleavages.  In  some  cases  narrow,  even  bands 
penetrate  a  crystal  nearly  from  one  end  to  the  other  like  twin  lamellae, 
while  in  other  instances  irregular  patches  of  chlorite  occur  in  the  hornblende. 
In  some  such  patches,  and  still  better  in  others  which  are  distributed  through 
the  groundmass,  and  may  or  may  not  represent  former  crystals  of  horn- 
blende, the  formation  of  epidote  may  be  followed;  its  prismatic  microlites 
are  to  be  seen  invading  the  chlorite,  just  as  in  the  McKihhen  Tunnel  diorites. 
Other  patches  of  chlorite  are  in  process  of  conversion  into,  or  substitution 
by,  calcite.  Where  this  change  goes  on  in  a  partially  decomposed  crystal 
of  hornblende,  the  central  portion  of  the  area  is  generally  occupied  by 
the  fresh  mineral  and  chlorite ;  whereas  the  calcite,  sometimes  accompanied 
by  a  little  quartz,  occupies  the  border  of  the  pseudomorph.     When  the 


DETAILED  DESCRIPTION  OF  SLIDES.  125 

substitution  of  calcite  for  chlorite  begins,  the  conversion  of  hornblende 
into  chlorite  seems  to  cease,  and  this  slide  shows  many  bright,  fresh  frag- 
ments of  hoi'nblende  embedded  in  calcite.  There  is  no  indication  that  the 
hornblende  tends  to  pass  directly  into  calcite.  Were  such  a  process  going 
on,  we  should  find  denticles  of  calcite  penetrating  the  hornblende.  Neither 
do  I  see  any  reason  to  suppose  that  the  epidote  in  this  slide  passes  into 
calcite;  it  appears  rather  to  give  place  to  clouds  of  dark-colored  opaque 
matter,  which  may  be  oxides  or  earthy  silicates.  In  this  and  the  other  slides 
of  andesite  which  contain  hornblende  free  of  black  borders,  I  see  no  indica- 
tion that  magnetite,  or  anything  resembling  magnetite,  results  from  the 
decomposition  of  hornblende.  In  the  black-bordered  hornblendes  I  have 
often  suspected  such  a  change,  but  I  see  no  way  of  proving  that  the  particles 
in  question  may  not  have  formed  a  part  of  the  original  border.  A  horn- 
blende in  process  of  decomposition  is  shown  in  Fig.  2,  Plate  II.,  from  this 
slide.  A  portion  of  the  augites  are  also  partially  converted  into  chlorite,  and 
in  the  pseudomorphs  epidote  is  certainly  developed  parasitically.  The  large 
feldspars  are  triclinic,  and  give  angles  of  extinction  answering  to  labradorite. 
In  one  of  them  a  bubble-bearing  and  only  partially  devitrified  glass  inclusion 
was  observed.  The  microlitic  groundmass  contains  some  magnetite,  p5'^rite, 
and  ordinary  apatite. 

Slide  116.    Crown  Point  Ravine. 

propyiitic  variety. — Tliis  is  a  Very  black,  fine-grained  rock,  which,  however, 
proves,  under  the  microscope,  to  derive  its  color  from  an  unusual  amount 
of  magnetite  in  the  groundmass.  The  hornblendes  are  altered  to  chlorite 
and  epidote,  and  only  a  few  sections  have  retained  characteristic  outlines. 
It  is  evident  that  the  chlorite  preceded  the  epidote,  and  in  some  cases  the 
encroachment  of  the  latter  can  be  very  well  observed.  A  portion  of  the 
chlorite  has  been  replaced  by  quartz  and  calcite. 

As  I  shall  have  occasion  to  refer  to  slides  of  the  Fortieth  Parallel  Sur- 
vey collection  from  Crown  Point  Ravine,  and  from  the  South  Twin  Peak, 
it  would  be  an  unnecessary  repetition  to  say  more  of  my  own  sections  from 
these  localities  than  that  there  is  no  notable  difference  between  them  and 
those  described  by  Professor  Zirkel. 


126  GEOLOGY  OF  THE  COMSTOCK  LODE. 

AUGITE-ANDESITE. 

Slide  122.    Peak  south  of  Crown  Point  Ravine,  marked  7075. 

Typical  variety. — Tliis  Is  a  blaclc,  rather  fine-grained,  apparently  crystalline 
rock,  with  a  somewhat  pitchy  luster.  Under  the  microscope  it  is  seen  to  be 
composed  of  augite  and  triclinic  feldspar,  with  apatite  and  magnetite  as 
accessory  constituents.  The  feldspar  is  sharply  angular,  but  there  is  no 
special  tendency  in  the  larger  crystals  to  elongation.  The  large  crystals 
give  very  high  angles  of  extinction,  many  of  them  exceeding  the  labradorite 
limits,  and  they  must  all  therefore  be  regarded  as  anorthite.  Among  the 
elongated  microlites  I  noticed  many  which  gave  too  high  an  angle  for  oligo- 
clase,  but  none  which  exceeded  the  labradorite  limits.  The  feldspars  contain 
partially  devitrified  glass  inclusions  and  augite  microlites.  The  greater  part 
of  the  augite  is  fresh.  It  is  very  light  brown  in  color  and  slightly  dichroitic. 
It  is  not  specially  well  crystallized,  and  shapeless  masses  are  more  abundant 
than  perfect  sections.  There  is  a  decided  tendenc}^  to  the  development  of 
only  one  of  the  prismatic  cleavages,  and  I  found  no  trace  of  pinacoidal 
cleavage.  There  are  numerous  bubble-bearing  glass  inclusions.  Manj^  of 
the  augites  are  converted  in  whole  or  in  part  into  chlorite,  of  the  same 
properties  mentioned  so  often  in  previous  descriptions.  There  is  a  single 
bright  brown  hornblende  of  small  size  heavily  bordered.  The  apatite  is  in 
part  colorless  and  in  part  brown.  The  magnetite  shows  no  peculiarities. 
The  groundmass  is  microlitic  and  in  parts  shows  a  felted  structure. 

Slide  137.    Bench  400  feet  southeast  of  intersection  of  Crown  Point  Eavine  and. 
Water  Company's  flume. 

The  same  slightly  decomposed. — This  is  macroscopically  and  microscopically  the 
same  rock  as  the  preceding,  being  merely  somewhat  more  decomposed.  The 
feldspars  contain  secondary  fluid  inclusions;  the  augite  is  wholly  converted 
into  chlorite,  which  for  the  most  part  retains  the  augitic  forms ;  and  epidote, 
quartz,  and  calcite  are  developing  from  the  chlorite. 

Slide  416.    First  peak  above  Ophir  Grade,  south  of  Crown  Point  Eavine. 

Variety  with  augitic  groundmass. — Tlus  Is  a  gray  porphyritic  rock,  with  none  of 
the  resinous  look  which  augite  rocks  usually  possess;  and  though  it  shows 


DETAILED  DESCRIPTION  OF  SLIDES.  127 

no  hornblende,  it  might  readily  be  mistaken  for  a  hornblende-andesite. 
Under  the  microscope  the  slide  shows  little  or  no  hornblende,  but  an  unusual 
amount  of  augite,  which  is  present,  not  only  as  porphyritical  crystals,  but  as 
microlites  in  the  groundmass  in  nearly  the  same  quantity  as  the  feldspar.  A 
majority  of  the  augites  are  fresh,  but  many  are  deco-mposed  to  chlorite,  which 
in  its  turn  is  largely  changed  to  epidote.  The  latter  may  be  seen  eating  its 
way  into  the  chlorite,  as  it  has  been  described  in  the  diorites  and  hom- 
blende-andesites.  Only  a  single  patch  of  chlorite  suggests  hornblende,  and 
there  is  none  of  that  mineral  in  a  fresh  condition.  The  feldspars  contain 
devitrified  glass  and  secondary  fluid  inclusions.  They  are  much  dimmed 
by  the  presence  of  chlorite  and  calcite. 

Slide  315.    Sutro  Tunnel,  1,400  feet  from  entrance. 

Variety  with  felt-like  groundmass. — Tliis  is  a  dark,  rcsiuous-looking  rock,  with 
some  large  greenish  feldspars.  The  slide  shows  many  fresh  augites  well 
crystallized,  somewhat  dichroitic,  and  with  a  tendency  to  develop  only 
one  cleavage.  Others  have  imdergone  a  somewhat  peculiar  decomposition, 
the  product  of  which  seems  to  be  chlorite,  very  heavily  charged  with 
hydrated  ferric  oxide.  There  are  few  augite  microlites  in  the  groundmass, 
but  many  in  the  feldspars.  The  feldspars  are  well  developed,  and  a  very 
few  only  give  angles  of  extinction  answering  to  anorthite,  in  spite  of  the 
fact  that  a  considerable  number  show  periclinic  twinning,  and  seem  to  be 
cut  nearly  in  orthopinacoidal  section.  A  good  many  such  give  almost 
exactly  the  theoretical  maximum  angle  of  extinction  of  labradorite,  and  I 
incline  to  the  belief,  for  which  there  is  no  substantial  pi'oof,  that  they  really 
belong  to  that  species,  and  that  consequently  both  of  the  more  basic  feld- 
spars are  present.  There  is  much  magnetite  and  many  dark  apatites.  The 
groundmass  is  a  felt-like  aggregation  of  tiny  microlites,  between  which 
there  is  certainly  a  small  amount  of  glass. 

Slide  481.    Between  summit  of  Mount  Kate  and  Occidental  Grade,  near  point  5639. 

Glassy  variety. — Tlils  Is  a  gray  glassy-lookiug  rock,  unlike  the  ordinary 
augite-andesite.  The  slide,  however,  shows  it  to  be  decidedly  of  that 
species,  and  to  consist  essentially  of  augite  and  triclinic  feldspar,  embedded 


128  GEOLOGY  OF  THE  COMSTOCK  LODE. 

in  a  true  colorless  glass.  A  few  small  hornblendes  and  some  magnetite  are 
the  subsidiary  minerals.  The  feldspars  show  little  tendency  to  elongation; 
they  appear  to  be  all  triclinic,  and  the  maximum  angles  of  extinction  ob- 
tained correspond  to  labradorite.  The  augites  are  not  very  sharply  crys- 
tallized, are  largely  massed  in  bunches,  and  are  more  than  ordinarily 
dichroitic.  There  are  a  few  hornblendes  which  are  bright  brown  in  color, 
and,  like  those  in  the  glassy  hornblende-andesite  of  the  District,  without 
black  borders.  The  glass  which  forms  a  large  part  of  this  rock  is  colorless, 
and  shows  in  places  perlitic  cracks.  Many  microlites  and  trichites  are  dis- 
tributed through  it,  some  of  them  transparent  and  very  likely  feldspathic; 
others  opaque.  Some  of  the  tiichites  show  a  beaded  structure.  Embedded 
in  the  glass  are  many  curious  spots  of  rounded  shape,  which  are  yellowish- 
white  by  reflected  light  and  feebly  transmit  yellow  rays.  In  polarized  light 
they  are  seen  to  be  wholly  or  partly  crystalline;  they  are  not  sufficiently 
diaphanous  to  say  which.  They  are  evidently  irregularly  radial  in  structure, 
and  in  some  favorable  instances  give  a  broad  ill-defined  cross  between 
crossed  Nicols.  The  pseudo-spherolitic  structure  is  further  marked  by  more 
or  less  curved  opaque  trichites  which,  starting  from  the  center,  preserve  an 
approximately  radial  direction,  branching  like  twigs  at  short  intervals  One 
of  these  masses  is  shown  in  Fig.  20,  Plate  III. 

Slide  125.     Above  the  Ophir  grade,  due  west  of  Belcher  hoisting- works. 

Granitoid  variety. — This  is  &  bluish-gray  grauular  rock,  looking  almost  like 
an  older  crystalline  species.  Under  the  microscope,  however,  it  reveals 
itself  as  merely  an  unusually  coarse-grained  augite-andesite.  The  ground- 
mass  is  granular.  A  portion  of  the  augites  are  fresh,  the  remainder  con- 
verted into  chlorite. 

Slide  465.    Crown  Point  Eavine ;  on  flume,  near  drainage. 

Specimen  showing  stages  of  decomposition. — Macroscopically  a  bluish-gray,  com- 
pact, and  rather  granular  rock,  without  macroscopically  visible  bisilicates. 

The  slide  affords  an  unusually  fine  opportunity  of  studying  the  decom- 
position of  augite-andesite.  It  happens  to  contain  a  large  proportion  of 
augites  in  octagonal  sections,  the  outlines  of  which  have  been  but  little  dis- 


DETAILED  DESCRIPTION"  OF  SLIDES.  129 

turbed  by  the  formation  of  decomposition-products.  Some  of  the  augites 
are  almost  unattacked,  and  show  thoroughly  characteristic  cleavages,  ex- 
tinctions, etc.  Others  are  partially  converted  to  chlorite,  and  yet  others 
are  wholly  replaced  by  the  uniaxial,  dichroitic,  green  mineral.  Some  of 
the  pseudoraorphs  are  partially  converted  into  epidote,  the  characteristic 
prismatic  sprouts  of  which  may  be  seen  penetrating-  the  chlorite.  A  tine 
example  is  illustrated  in  Fig.  5,  Plate  II.  In  some  other  cases  the  degen- 
eration to  epidote  and  to  calcite  is  going  on  in  the  same  chloritic  pseudo- 
morph.  There  were  originally  one  or  two  small  hornblendes  in  this  slide, 
now  wholly  converted  into  epidote.  The  feldspars  seem  to  be  labradorite. 
Crown  Point  Ravine  is  the  best  of  all  the  "propylite"  localities,  and  the 
specimen  is  an  excellent  representative  of  the  rocks  which  have  received 
this  name.  A  portion  of  the  slide  is  very  faithfully  illustrated  in  Fig.  31, 
Plate  V. 

Slide  428     500  feet  southeast  of  Sutro  Tunnel  air-shaft. 

Specimen  with  peculiar  augites. — TWs  is  a  black  rock  with  au  uncven  fracture, 
and  a  luster  both  vitreous  and  resinous  Under  the  microscope  it  is  seen  to 
be  a  fine  augite-andesite  with  more  augite  than  usual,  and  no  hornblende. 
The  augite  is  of  the  common  color  and  slightly  dichroitic.  One  crystal 
shows  the  uncommon  phenomenon  of  multiple  twinning,  in  which  the  surface 
of  composition  of  a  portion  of  the  lamellae  is  decidedly  irregular.  This 
augite  is  illustrated  in  Fig.  1 6,'  Plate  III.  The  large  feldspars  give  angles 
of  extinction  corresponding  to  labradorite;  the  microlites  correspond  to 
oligoclase,  and  many  of  them  show  a  tendency  to  fibration  at  the  ends. 
The  large  feldspars  contain  inclusions  of  glass  and  microlites  of  augite. 
There  are  a  few  brown  apatites  and  some  colorless  ones  in, the  slide.  The 
groundmass  has  the  well-known  felted  appearance  in  some  portions  and 
shows  tluidal  arrangement  in  others.  It  contains  a  considerable  amount  of 
isotropic  glass. 

Slide  31.     Sutro  Tunnel,  10,055  feet  from  entrance. 

Specimen  with  unusual  chlorite  pseudomorph. — This  is  ail  Ordinary  augite-audcslte 
in  a  somewhat  decomposed  condition,  which  most  likely  carried  a  little 

9  0  L 


130  GEOLOGY  OF  THE  COMSTOCK  LODE. 

glass  when  fresh.  Among  the  many  pseudomorphs  of  chlorite  after  augite 
which  it  contains,  one  is  especially  beautiful,  and  is  illustrated  in  Fig.  4, 
Plate  II.  Decomposition  has  evidently  started  from  the  cross-fractures, 
and  also  from  the  centers  of  the  fragments  isolated  by  the  o'acks,  but  these 
two  varieties  of  decomposition  have  proceeded  somewhat  differently.  The 
chlorite  around  the  exterior  of  the  crystal,  and  along  the  cracks,  betrays 
structure  only  by  a  very  slight  dichroism ;  between  crossed  Nicols  it  is  not 
perceptibly  luminous.  The  chlorite  which  has  developed  from  the  cen- 
ters of  the  fragments  is  brownish-green,  radially  fibrous,  strongly  dichroitic, 
and  polarizes  in  dull-brownish  colors. 


LATER   HORNBLENDE-ANDESITE. 

Slides  472  and  473.    Quarry  2,000  feet  northeast  of  Sutro  Tunnel  Shaft  III. 

Trachytic-iooking  variety. — TWs  is  a  Very  coarse-grained  soft  rock,  with  large 
porphyritical  feldspars  and  visible  mica  and  hornblende.  Its  ground  mass  is 
purplish-gray.  This  rock  is  that  commonly  employed  on  the  Comstock  for 
engine  foundations  and  the  like. 

Under  the  microscope  it  is  seen  to  consist  of  jjlagioclase,  hornblende, 
mica,  and  magnetite,  with  a  few  Carlsbad  twins  and  apparently  simple 
crystals  which  might  be  sanidin.  Some  of  these  last  show  minute  stripes 
under  close  examination,  and  others  can  be  shown  to  be  plagioclase  by 
their  angles  of  extinction.  The  highest  angles  of  extinction  of  the  properly 
oriented  feldspars  indicate  labradorite  ;  but  many  of  the  larger  crystals  show 
a  strongly  marked  zonal  structure.  Inferences  as  to  their  composition 
have  been  drawn  on  page  68.  The  feldspathic  microlites  appear  to 
be  oligoclase.  The  mica  is  brown  and  intensely  dichroitic.  Cleavage- 
scales  give  an  hyperbolic  interference  figure,  which  could  not  for  a  moment 
be  confounded  with  a  cross,  and  it  is  either  a  biotite  in  which  the  angles  of 
extinction  are  uncommonly  large  or  another  species  The  hornblendes 
are  brown  and  well  crystallized,  and  are  all  black-bordered;  but  while  some 
have  comparatively  narrow  borders,  others  are  almost  wholly  converted  to 
magnetite,  leaving  only  a  particle  of  the  fresh  mineral  near  the  center.     The 


DETAILED  DESCRIPTION  OF  SLIDES.  131 

same  remark  applies  to  the  mica.  Some  of  the  hornblende  crystals,  too, 
are  decomposed,  while  the  majority  are  perfectly  fresh.  This  is  probably 
due  to  the  structure  of  the  rock,  which  must  admit  liquid  currents  more 
easily  on  certain  lines  than  on  others.  The  grouudmass  is  thoroughly 
crystalline  and  microlitic,  and  consists  of  feldspar  microlites  and  magnet- 
ite. This  is  the  most  important  of  the  so-called  trachytes  of  the  District, 
and  was  therefore  selected  for  illustration.  Plate  V.,  Fig.  '62,  shows  a 
characteristic  field. 

Slide  474.    Quarry  2,000  feet  east  of  Occidental  Mill. 

A  similar  rock. — Thls  is  a  reddish  porphyry  similar,  except  in  color,  to  that 
described  under  slide  472,  and  also  used  for  building  purposes.  Under  the 
mici'oscope  it  is  remarkable  for  its  intensely  dichroitic  hornblende,  which 
shows  an  extremely  light  yellowish-brown  tint,  when  parallel  to  the  long 
section  of  the  analyzer,  and  a  bright  red-brown  when  at  right  angles  to  this 
position.  The  slide  also  contains  considerable  poorly  crystallized  augite. 
The  mica  gives  the  same  decidedly  biaxial  interference  figure  as  that  in 
slide  472.  The  feldspars  also  are  similar  to  those  in  that  slide.  This  is 
the  rock  separated  by  Dr.  Hawes  by  Thoulet's  method,  and  found  to  con- 
tain no  sanidin. 

Slide  230.     Quarry  above  Utah  mine. 

More  compact,  gray  rock. — A  bluish-gray  Tock  of  manifestly  loose  texture, 
showing  both  mica  and  hornblende.  Under  the  microscope  plagioclase, 
magnetite,  and  some  brown  glass  are  also  visible.  The  feldspars  are  beau- 
tifully fresh.  Extremely  few  lack  stripes,  and  these  are  not  in  determin- 
able zones.  Several  of  the  larger  feldspars  show  nearly  square  sections  and 
pericline  as  well  as  albite  twinning.  These  give  angles  of  extinction  of 
about  30°  on  each  side  of  the  albitic  twinning  plane.  The  small  elongated 
feldspars  also  give  labradorite  angles  in  many  cases,  and  I  see  no  reason  to 
.  suspect  any  considerable  quantity  of  any  other  feldspar.  The  feldspars  con- 
tain great  numbers  of  colorless  glass  inclusions,  most  of  them  entirely  fresh, 
as  well  as  patches  of  the  brown  glass  and  of  groundmass  with  glass.  A  few 
of  the  smaller  feldspars  seem  to  consist  of  negative  crystals  of  brown  glass 


132  GEOLOGY  OF  THE  GOMSTOOK  LODE. 

surrounded  by  thin  shells  of  feldspar.  The  hornblende  is  in  part  perfectly 
fresh,  and  so  solid  that  the  cleavages  are  almost  imperceptible  with  low  powers. 
The  color  of  the  hornblendes  is  various,  and  seems  to  depend  largely  on  their 
position.  Some  crystals  are  nearly  pure  brown;  others  a  slightly  brownish- 
green,  or  of  intermediate  tints.  A  few  ai-e  decomposed  to  calcite,  quartz,  and 
epidote.  A  part  of  the  crystals  show  no  black  borders,  and  others  only  a 
very  narrow  line  of  magnetite.  The  mica  is  fresh  biotite,  giving  the  char- 
acteristic interference  figure,  and  like  the  hornblende  shows  a  few  glass 
inclusions.  It  has  a  narrow  black  border.  The  groundmass  is  composed  of 
feldspar  microlites  and  brown  glass.  In  parts  of  the  slide  the  arrangement 
of  the  microlites  seems  wholly  without  order,  while  in  others  fluidal  structure 
is  well  developed. 

Slide  462.    2,000  feet  northwest  of  Geiger  Grade  Toll  House. 

Black,  glassy  variety. — This  is  a  pitcliy-black  rock,  with  a  glassy  luster,  showing 
some  large  hornblendes,  but  resembling  certain  augite-andesites  in  appear- 
ance more  than  any  hornblende-andesite  of  the  District.  Under  the  micro- 
scope the  reason  of  this  unusual  appearance  is  plain,  for  it  contains  a  large 
amount  of  glass  base,  which  is  not  the  case  with  any  other  Washoe  rock 
of  this  species  examined.  Nearly  all  the  feldspars  seem  to  be  labradorite, 
only  the  minutest  untwinned  microlites  giving  angles  of  extinction  proper 
to  oligoclase.  A  few  sections  give  angles  of  extinction  which  might  be  re- 
ferred to  anorthite,  but  I  failed  to  find  any  such  in  which  extinction  took 
place  at  equal  angles  to  the  trace  of  the  twinning  plane,  and  suppose  the 
crystals  to  be  labradorites  cut  in  one  of  the  uninvestigated  zones.  Many 
of  the  feldspars  at  first  sight  appear  to  be  simple  crystals,  but  show  on 
closer  examination  a  few  exceedingly  minute  strise.  The  feldspars  contain  a 
very  unusual  abundance  of  glass  inclusions,  a  large  proportion  of  which 
have  polygonal  outlines  parallel  to  the  sides  of  the  feldspar  section.  They 
also  contain  inclusions  of  the  base,  many  of  which  assume  fantastic  forms, 
some  looking  like  ripple-marks,  and  others  arranged  as  if  the  base  had  pene- 
trated perpendicularly  into  the  feldspar  and  spread  between  its  zones.  The 
process  must  have  been  just  the  reverse,  and  the  appearance  is  no  doubt 
due  to  an  ineffectual  effort  of  the  feldspathic  material  to  free  itself  from  the 


DETAILED  DESCBIPTION  OF  SLIDES.  133 

adhesive  glass  during  crystallization.  Some  of  these  inclusions  are  shown 
in*  Fig.  23,  Plate  III.  Zonal  structure  is  beautifully  developed  in  many  of 
these  feldspars. 

The  hornblendes  are  of  a  somewhat  dull  yellowish-green  color  and 
very  solid.  They  are  especially  remarkable  from  the  fact  that  scarcely 
any  of  them  show  even  a  trace  of  a  black  border.  There  is  a  large  quan- 
tity of  augite  of  exceptionally  pale  color.  It  is  faintly  dichroitic  and 
crystallized  in  unusually  long  needles.  The  sections  and  angles  of  extinc- 
tion leave  no  doubt  as  to  its  nature.  There  is  a  single  excellent  biotite  in 
the  slide.  Many  colorless  apatites  and  a  considerable  quantity  of  magnetite 
are  present.  The  base  shows  a  felt-like  structure  which  appears  to  be  due 
to  the  presence  of  minute  opaque  microlites. 

Slide  470.   Mount  ^.bbie. 

Gray  coarse-grained  variety. — Thls  Is  a  coarsc  gray  porous  rock,  strougly  re- 
sembling a  hornblende-trachyte  in  appearance.  Under  the  microscope, 
however,  it  is  plain  that  nearly  or  quite  all  the  feldspars  are  triclinic,  and 
they  give  labradorite  angles  of  extinction.  Many  of  the  smaller  labrador- 
ites  are  simple  individuals.  The  hornblende  and  augite  are  such  as  are 
common  in  the  hornblende-andesites,  but  the  hornblendes  show  only  very 
narrow  black  borders.  There  are  about  twice  as  many  hornblendes  as 
augites.     The  groundmass  is  microlitic,  and  no  base  is  visible. 

Slide  467.    1,000  feet  nortli-northwest  of  Flowery  Peak. 

Porphyry  with  dark  groundmass. — This  is  a  coarse-grained  rock  in  which  large 
crystals  of  feldspar  are  separated  out  in  a  dark,  rather  compact  groundmass. 
Mica  as  well  as  feldspar  is  visible.  Most  of  the  feldspar  crystals  show  twin 
striations,  but  there  are  some  Carlsbad  twins  and  simple  crystals,  none  of 
which,  however,  are  probably  orthoclastic.  Many  of  the  larger  crystals, 
which  are  well  developed,  show  zonal  structure.  The  maximum  angles  of 
extinction  correspond  to  labradorite.  The  feldspathic  microlites  are  shorter 
and  broader  than  is  usual,  and  a  few  are  possibly  sanidin,  but  the  great 
majority  are  cei'tainly  triclinic.  I  noticed  no  inclusions  in  the  feldspars 
beyond  apatite.     Hornblende  and  mica  are  almost  wholly  represented  by 


134  GEOLOGY  OF  THE  COMSTOCK  LODE. 

patches  of  magnetite  which  are  evidently  exaggerated  black  borders. 
Some  of  these  patches  are  pseudomorphs  after  hornblende,  but  mica  plainly 
predominated.  Of  this  enough  is  left  to  determine  that  its  color  was  brown. 
The  slide  contains  a  single  grain  of  quartz,  which  is  evidently  primary. 
The  groundmass  is  thoroughly  crystalline  and  microlitic.  It  contains  much 
magnetite,  and  shows  fluidal  structure  in  places. 

Slide  476.    Divide  between  Mount  Eose  and  Mount  Emma. 

Light-gray  porous  variety. — A  light-gray  rock  with  a  large  amount  of  visible 
mica  and  hornblende.  The  feldspar  is  largely  in  simple  crystals,  few,  if 
any,  of  which,  however,  are  orthoclastic.  The  microlites  are  developed 
with  unusual  sharpness.  The  feldspars  contain  bubble-bearing  glass  inclu- 
sions and  patches  of  groundmass.  Hornblende  is  much  more  abundant  in 
this  slide  than  mica,  and  is  remarkable- for  the  fact  that  it  shows  scarcely  a 
trace  of  black  border.  Much  of  it  occurs  as  minute  brown  spiculse  dis- 
seminated through  the  groundmass.  The  mica  is  present  in  well-developed 
crystals,  and  cleavage  scales  show  the  ordinary  sensibly  uniaxial  interfer- 
ence figure  of  biotite.  The  iron  ore  is  magnetite,  and  it  and  the  feldspar 
microlites  of  the  groundmass  seem  to  be  imbedded  in  a  colorless  glass. 


BASALT. 
Slide  457.    Basalt  mesa,  just  west  of  Silver  City. 

Only  variety. — Thls  Is  a  dark  Ordinary  basalt,  with  numerous  visible  fresh 
amber-colored  olivines.  Under  the  microscope  it  is  seen  to  be  a  crystalline 
mixture  of  olivine,  augite,  feldspar,  and  magnetite.  The  olivine  is  nearly 
colorless  in  the  section,  but  has  a  faint  yellowish  tinge.  It  occurs  for  the 
most  part  in  grains  showing  only  one  or  two  •  crystalline  faces  or  none  at 
all.  A  few  of  the  larger  crystals  have  good  hexagonal  or  octagonal  out- 
lines. Besides  irregular  cracks,  there  are  occasional  indications  of  imper- 
fect cleavage.  The  olivine  is  wholly  undichroitic  and  polarizes  brilHantly. 
As  inclusions  it  contains  crystals  of  magnetite  and  a  few  particles  of  augite. 
It  shows  only  occasional  traces  of  decomposition.  The  augite  is  of  the 
common  brown  color,  but  of  a  rather  deeper  tint  than  usual.     Some  of  the 


PROPYLITES  OF  THE  FOETIETH  PARALLEL.  135 

crystals  are  as  large  as  the  olivines,  but  while  there  are  many  small  augites 
there  seem  to  be  no  microscopic  olivines.  The  augites  are  better  crystal- 
lized than  the  olivine,  and  often  show  characteristic  sections  and  cleavages. 
They  are  decidedly  dichroitic.  The  aiigites  carry  a  few  bubble-bearing 
inclusions,  which  seem  to  be  glass.  The  feldspars  are  small,  lath-like,  and 
often  simply  twinned.  In  a  very  few  cases  both  periclinic  and  albite  twin- 
ning are  visible.  The  angles  of  extinction  are  those  of  labradorite.  I  could 
detect  no  orthoclase.  The  groundmass  consists  of  feldspar,  augite,  and 
magnetite  in  cubes,  and  contains  no  perceptible  base.  Slide  458  from  1,250 
feet  southeast  of  Roux's  Ranch  is  identical  with  the  above,  and  with  the  slide 
described  in  the  Exploration  of  the  Fortieth  Parallel,  Vol.  VI.,  as  528.  The 
slide  and  specimen  described  in  that  memoir  as  529  is  the  same  rock  which 
is  here  regarded  as  a  metamorphic  diorite. 


PROPYLITES  OF  THE  FORTIETH  PARALLEL  SURVEY  COLLECTION. 

Fortieth  Parallel  propyiites. — I  havc  bceu  kiudly  allowcd  free  use  of  the  collec- 
tions of  the  Geological  Exploration  of  the  Fortieth  Parallel,  and  reproduce 
in  the  following  pages  my  notes  on  the  specimens  and  slides  described  in 
Vol.  VI.  of  the  publications  of  that  survey  as  propyiites  (212  to  225)  and 
as  quartz-propylites  (226  to  232).  While  I  do  not  feel  myself  competent 
to  decide  definitively  the  species  of  those  rocks  which  I  have  not  had  an 
opportunity  of  studying  in  the  field,  my  opinion  of  each  slide  is  indicated, 
in  order  to  convey  a  more  complete  impression  of  its  appearance. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  212,  specimen  No.  22,682,  Crown  Point 
Eavine,  Washoe. 

This  is  a  smooth,  fine-grained  rock,  somewhat  resembling  a  limestone 
in  texture.  Its  color  is  pistachio  green.  Seen  under  the  microscope,  it  is 
evidently  much  decomposed;  indeed,  the  slide  shows  little  besides  epidote 
and  secondary  quartz.  Even  the  magnetite  has  almost  wholly  disappeared, 
and  the  residual  products  are  grouped  within  no  outlines  from  which  the 
nature  of  the  original  bisilicates  might  be  inferred  Many  very  small  feld- 
spars are  still  fresh  enough  to  make  out  with  certainty  that  they  are  triclinic. 


136  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  213,  specimen  No.  22,684,  Crown  Point 
Eavine,  Washoe. 

A  somewhat  more  granular  rock  than  the  preceding,  but  of  the  same 
color.  The  slide  shows  that  it  is  slightly  less  decomposed.  In  a  few  cases 
feldspars  can  be  detected  with  striations  not  entii-ely  obliterated,  and  with 
rectilinear  outlines,  such  as  are  ordinarily  met  with  in  andesites.  Several 
brown  apatites  are  visible.  The  patches  of  decomposition  products  show 
outlines  here  and  there  which  are  suggestive  of  hornblende  and  augite. 
Besides  quartz  and  epidote,  this  slide  contains  some  calcite.  I  regard  this 
and  the  preceding  rock  as  entirely  indeterminable  froiii  the  specimens  and 
slides,  but  from  a  study  of  their  associations  on  the  spot  T  believe  them  to 
be  hornblende-andesites. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  214,  specimen  No.  22,686.     Crown 
Point  Eavine,  Washoe. 

A  gray  coarse-grained  rock,  the  feldspars  of  which  are  opaque,  giving 
it  a  superficial  resemblance  to  pre-Tertiary  rocks.  Under  the  microscope  a 
glance  shows  it  to  be  augitic.  The  slide  contains  several  sections  of  the 
undecomposed  mineral  with  characteristic  octagonal  outlines  and  appropriate 
angles  and  cleavages,  as  well  as  some  longitudinal  sections,  giving  angles 
of  extinction  running  up  to  above  30°.  The  color  of  this  augite  is  the  com- 
mon brownish-yellow,  not  unlike  the  tint  of  bamboo.  Much  of  the  augite 
has  been  decomposed  to  chlorite  of  fibrous  structure,  which  shows  dark 
bluish  tints  between  crossed  Nicols,  aggregate  and  sometimes  spherolitic 
polarization,  and  extinction  when  the  microlites  are  parallel  to  the  principal 
sections  of  the  Nicols.  That  the  chlorite  is  a  derivative  of  the  augite  is  clear, 
for  in  some  cases  augites  are  only  in  part  converted  into  chlorite,  and  in 
others  the  pseudomorphs  are  perfect,  even  retaining  traces  of  the  cross- 
fractures  of  the  augite  j^risms.  I  found  but  one  mass  of  decomposition 
products  that  might  with  any  probability  be  referred  to  hornblende.  The 
feldspars  are  tricliuic,  and  some  of  the  large  crystals  show  labradorite 
angles  of  extinction.  Apatite  and  magnetite  are  also  present.  The  gi'ound- 
mass  contains  no  glass,  but  seemed  to  me  to  show  traces  of  a  felt-like  struct- 
ure,  much  obscured,  however,  by  particles  of  chlorite  and  epidote.     This 


PEOPYLITBS  OF  THE  FOETIETH  PARALLEL.  137 

rock  is  an  augite-andesite,  and  one  characteristic  of  the  District,  but  is 
partially  decomposed. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  215,  specimen  No.  22,689.     Crovm 
Point  Eavine,  Washoe. 

A  very  dark,  somewhat  basaltic-looking  rock.  The  slide  resembles 
that  last  described,  containing,  however,  only  pseudomorphs  of  chlorite  after 
augite,  and  none  of  the  fresh  mineral.  The  chlorite  and  the  mineral  from 
which  it  was  derived  were  carefully  identified  in  the  manner  indicated  in 
the  last  paragraph.  There  is  no  fresh  hornblende,  but  a  few  very  minute 
oval  rings  of  magnetite  grains  probably  represent  the  black  borders  of 
former  hornblendes.  The  feldspars,  which  are  not  distinguishable  from 
ordinary  andesitic  plagioclases,  contain  spots  which  look  like  devitrified 
glass-inclusions.     This,  too,  is  augite-andesite. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  216,  specimen  No.  22,690,  from  Crown 
Point  Eavine,  Washoe. 

This  rock  is  much  decomposed,  and  neither  augite  nor  hornblende  are 
present  in  a  fresh  state,  but  the  slide  contains  many  black  borders,  which 
retain  the  characteristic  outlines  of  hornblende,  though  they  now  surround 
only  calcite,  quartz,  and  a  few  residual  grains  of  epidote.  There  is  also 
one  good  pseudomorph  of  chlorite  after  augite.  From  my  acquaintance  with 
the  rocks  of  the  District  I  have  no  hesitation  in  pronouncing  this  a  horn- 
blende-andesite. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  217.     Gold  Hill  Peak,  Washoe. 

There  is  no  specimen  in  the  collection  corresponding  to  this  slide  or 
to  the  locality,  which  is  represented  on  the  map  accompanying  this  paper 
by  the  southern  "Twin  Peak".  My  own  specimens  are  coarse  greenish- 
gray  rocks  of  somewhat  open  texture.  The  feldspars  are  not  thoroughly 
transparent  in  consequence  of  incipient  decomposition.  The  fresher  por- 
tions of  the  mass  show  brilliant  hornblendes.  The  slide  contains  some  fresh 
brown  hornblendes  with  black  borders,  and  some  black  borders  from  which 
the  bisilicate  has  disappeared.  A  portion  of  the  hornblende  exhibits  the 
intermediate  color  between  green  and  brown,  which  is  seen  in  so  many 


138  GEOLOGY  OF  THE  COMSTOCK  LODE. 

brown  hornblende  rocks;  but  I  failed  to  find  green  hornblende,  fibrous  horn- 
blende, or  hornblende  without  a  black  border.  There  are  a  few  excellent 
augites  and  many  capital  pseudomorphs  of  chlorite  after  augite.  This  chlorite 
shows  the  usual  structure,  dichroism,  extinction  parallel  to  the  fibers,  etc. 
The  feldspars  are  triclinic,  the  large  ones  seemingly  labradorite,  and  they 
appear  to  contain  devitrified  gla^s  inclusions.  There  are  many  brown  and 
dusty  apatites.  The  groundmass  has  the  microlitic  structure  of  hornblende- 
andesites,  nor  can  I  see  any  reason  for  separating  this  rock  from  that  species. 

Exploration  of  the  Fortieth  Parallel.     Slides  Nos.  218  and  219,  specimen  No.  22,694. 
Ophir  Eavine,  Washoe. 

These  slides  I  have  sufficiently  discussed  in  describing  my  own  thin 
sections  from  the  same  locality.  I  have  there  considered  the  rock  as  a  dio- 
rite-porphyry. 

Exploration  of  the  Fortieth  Parallel.     SKde  No.  220,  specimen  No.  22,588.    Hill  east 
of  Steamboat  Valley,  Virginia  Eange. 

This  is  a  brown  rock  which  looks  like  an  impure  limonite.  Under  the 
microscope  nothing  is  visible  excepting  ferric  hydrate  and  a  little  secondary 
quartz. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  221,  specimen  No.  22,574.     Sheep 
Corral  Canon,  Virginia  Eange. 

This  is  a  light  greenish,  granular  r^ck,  evidently  composed  of  feldspar 
and  hornblende.  The  slide  shows  that  the  hornblende  is  wholly  decom- 
posed. The  crystals  of  this  nnneral  appear  to  have  had  black  borders, 
which  are  now  in  part  replaced  by  higher  oxides.  When  fresh  it  contained 
great  numbers  of  small  augites,  which  are  now  converted  into  the  ordinary 
chlorite.  The  same  product  of  decomposition  is  also  disseminated  through 
the  groundmass,  and  is  accompanied  by  quartz  and  calcite.  The  feldspar 
is  fresh  and  striated,  and  the  general  character  under  the  microscope  is  that 
of  an  andesite.  I  can  see  no  reason  for  calling  it  anything  but  hornblende- 
andesite. 

Professor  Wiedemann  analyzed  this  rock  and  found  64.62  per  cent.  siUca. 
In  discussing  this  analysis  the  fact  should  not  be  overlooked  that  a  relative 
increase  in  the  quantity  of  silicic  acid  commonly  accompanies  decomposition. 


PEOPYLITES  OP  THE  FORTIETH  PARALLEL.  139 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  221*,  specimen  No.  21,950.    Between 
the  Truckee  and  Montezuma  Ranges. 

The  specimen  strongly  resembles  those  from  the  head  of  Ophir  Ravine, 
Washoe.  It  is  highly  decomposed,  but  the  bisilicates  appear  to  have  been 
hornblende.  The  feldspars  have  not  the  sharp  outlines  usual  in  andesites, 
and  the  toute  ensemble  is  that  of  a  porphyritic  diorite. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  222,  specimen  No.  21,542.     Storm 
Canon,  Fish  Creek  Mountains. 

This  is  a  rather  coarse-grained  greenish  rock,  in  which  lath-like  feld- 
spars show  prominently  in  a  finer  groundmass.  The  slide  contains  an 
abundance  of  augites,  some  of  which  show  pinacoidal  cleavages  as  well  as 
the  prismatic  ones.  The  cleavages  are  very  heavily  marked.  A  portion  of 
the  augite  has  been  converted  into  grayish-green  uralite,  distinctly  retaining 
the  crystal  form  of  augite.  Where  it  is  favorably  oriented  it  gives  angles 
of  extinction  of  about  15°.  The  greater  part  of  the  augite  has  degen- 
erated into  chlorite,  with  the  usual  structure  and  optical  properties.  The 
slide  further  contains  much  fresh  mica,  some  of  the  scales  of  which  are 
horizontally  placed,  and  give  the  biotite  interference  figure.  There  'is  also 
a  very  little  brown  and  intensely  dichroitic  hornblende,  but  I  could  find 
none  of  this  mineral  which  was  green,  except  the  uralite.  The  larger 
plagioclases  are  well  developed  in  lath-like  crystals,  but  are  nearly  opaque 
in  consequence  of  the  presence  of  "decomposition  products..  The  iron  ore 
occurs  in  irregular  masses,  but  its  nature  is  uncertain.  The  groundmass  is 
granular,  not  composed  of  well-developed  microlites,  but  thoroughly  crys- 
talline.    It  contains  much  epidote  and  chlorite. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  223,  specimen  No.  21,545.     Storm 
Canon,  Fish  Creek  Mountains. 

This  is  the  same  rock  as  the  last,  but  in  a  different  stage  of  decompo- 
sition. The  green  hornblende  shows  in  numerous  cases  the  crystal  outlines 
of  augite,  and  in  my  opinion  i_s  exclusively  uralite.  The  plagioclases  are 
fresher  than  in  the  other  slide  and  contain  rounded  fluid  inclusions  of  small 
size.     While  it  may  be  somewhat  rash  to  decide  upon  the  age  of  this  rock 


140  GEOLOGY  OP  THE  COMSTOCK  LODE. 

from  these  two  specimens  and  slides,  all  the  diagnostic  points  appear  to 
me  to  indicate  diabase  rather  than  augite-andesite  as  the  proper  determina- 
tion. 

Exploration  of  the  Fortieth  ParalleL     Slide  No.  224,  specimen  ISo.  21,259.     Foothills 
north  of  Tuscarora,  Gortez  Eange. 

This  is  a  green  porphyry,  with  impellucid  feldspars  and  brilliant  horn- 
blendes. I  am  almost  inclined  to  doubt  that  this  can  be  the  slide  described 
in  the  "Microscopical  petrography;"  but  the  dark  brown  hornblendes  tally 
precisely  with  the  figure  and  the  description,  the  slide  corresponds  to  the 
specimen,  as  does  the  latter  with  the  locality,  and  no  other  slide  labeled 
"propylite"  bears  any  considerable  resemblance  to  the  text  and  the  illustra- 
tion. The  slide  contains  a  large  number  of  unusually  symmetrical  brown 
hornblende  sections,  with  broad  black  borders.  One  or  two  of  these  exhibit 
clinopinacoidal  cleavage  as  well  as  the*  usual  prismatic  one.  Many  of  the 
hornblendes  are  altered  into  chlorite,  still  retaining  the -black  border  and 
crystal  outlines.  This  chlorite  shows  the  usual  aggregate  polarization  in 
some  places  and  spherolitic  structure  in  others,  and  extinguishes  light  par- 
allel to  the  direction  of  the  principal  Nicol  sections.  There  are  also  many 
fresh  augites  with  characteristic  sections,  cleavages,  and  optical  properties, 
and  pseudomorphs  of  chlorite  after  augite.  As  usual  in  decomposed  rocks, 
the  groundmass  contains  irregular  patches  of  chlorite,  the  properties  of 
which  are  identical  with  those  of  that  in  pseudomorphic  forms.  I  could 
find  nothing  whatever  corresponding  to  the  green  hornblendes  described  by 
Professor  Zirkel,  and  figured  as  without  black  borders,  and  as  showing 
hornblendic  cleavages  and  outlines.  The  feldspars  and  groundmass  are  like 
those  usually  found  in  partially  decomposed  hornblende-andesite,  and  as 
such  I  have  no  hesitation  in  regarding  the  rock. 

Exploration  of  the  Fortieth  Parallel.     SUde  No.  225,  specimen  No.  21,314.    Wagon 
Canon,  Cortez  Eange. 

This  is  a  reddish  rock,  with  well-developed  porphyritic,  impellucid 
feldspars,  visible  mica,  and  greenish  black  patches,  which  are  possibly 
hornblendes.  Under  the  microscope  the  rock  is  seen  to  be  greatly  decom- 
posed.    The  slide  contains  fresh  mica,  numerous  pseudomorphs  of  chlorite 


PEOPYLITES  OF  THE  FORTIETH  PARALLEL.  141 

after  augite,  and  a  number  of  patches  of  chlorite,  which  seem  referable  with 
some  probability  to  hornblendic  forms.  The  feldspars  are  triclinic  and  very 
closely  striated.  None  of  the  angles  of  extinction  which  I  observed  exceeded 
the  oligoclase  limits.  The  slide  contains  very  little  epidote,  but  the  feldspars 
and  groundmass  are  clouded  with  calcite  and  limonite.  While  no  satisfac- 
tory determination  can  be  made  of  this  specimen,  it  seems  to  answer  best 
to  a  micaceous  hornblende-andesiteJ 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  226,  specimen  No.  21,604.    Hills  east 
of  Golconda  Station. 

Macroscopically  this  rock  is  of  a  greenish-gray  color  tinged  with  yel- 
low, and  shows  porphyritical  crystals  of  mica,  hornblende,  and  impellucid 
feldspars.  Under  the  microscope  it  is  apparent  that  the  feldspars  are  rend- 
ered almost  opaque  by  excessively  fine  grains  of  what  is  seemingly  calcite. 
Some  of  them  are  triclinic,  others  appear  to  me  to  be  orthoclase,  but  which 
are  in  the  majority  it  is  impossible  to  say.  The  rock  contains  quartz  in 
which  there  are  numerous  fluid  inclusions,  some  of  them  containing  carbonic 
acid.  The  quartz  also  carries  unquestionable  glass  inclusions  of  good  size, 
in  which  devitrification  has  proceeded  only  so  far  that  between  crossed  Nicols 
one  or  two  bright  points  appear  on  the  jet-black  ground  of  the  isotropic 
substance.  One  of  these  is  accompanied  by  the  short  cracks  in  the  quartz, 
which  have  often  been  observed,  and  which  so  beautifullv  illustrate  the 
elasticity  of  silica.  One  of  the  numerous  apatites,  too,  contains  a  glass 
inclusion  hung  like  a  drop  on  an  inclosed  microlite  which  is  probably 
also  apatite.  The  hornblende,  and  even  the  mica  are  wholly  replaced  by 
decomposition  products,  largely  oxides  of  iron.  I  could  detect  no  trace  of 
augite.  The  groundmass  contains  some  particles  of  epidote  and  chlorite. 
It  is  nearly  impossible  to  determine  a  rock  so  thoroughly  decomposed  with- 
out a  study  of  its  occurrence.  If  the  feldspar  is  triclinic  it  must  be  a  dacite, 
for  the  glass  precludes  the  supposition  that  it  is  a  diorite.  The  absence  of 
augite  and  of  well-developed  feldspar  microlites,  the  appearance  of  the 
orthoclase-like  larger  feldspai's,  the  abundance  of  fluid  inclusions,  and  the 
general  air  of  the  rock,  seem  to  put  dacite  almost  out  of  the  question.  Sim- 
ilar arguments  hold  against  its  determination  as  rhyolite,  and  but  for  the 


142  GEOLOGY  OF  THE  COMSTOCK  LODE. 

presence  of  carbonic  acid  in  the  inclusions,  which  is,  at  all  events,  very 
rare  in  quartz-porphyry,  I  should  class  it  as  a  member  of  that  group. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  227,  specimen  No.  21,500.    West  Gate, 
Augusta  Mountains. 

Macroscopically  a  gray,  granular  rock.  Under  the  microscope  it  bears 
a  strong  resemblance  to  the  preceding. ,  The  feldspars  are  almost  opaque, 
the  hornblende  and  mica  are  wholly  decomposed.  The  groundmass  con- 
tains some  epidote  and  much  chlorite,  magnetite,  and  zircon.  More  than 
one  of  the  quartzes  carry  besides  fluid  inclusions,  typical,  fresh,  colorless 
glass  inclusions  which  contain  bubbles  and  are  of  sufficient  size  to  remain 
black  between  crossed  Nicols.  I  noticed  an  apatite  with  good  prismatic 
cleavages.     This  rock  seems  to  me  an  old  quartz-porphyry. 

Exploration  of  the  Fortieth  Parallel.     Slides  Nos.  228  and  229,  specimen  No.  21,308. 
Cortez  Peak,  Cortez  Range. 

I  entirely  assent  to  Professor  Zirkel's  description  of  these  slides.  The 
rock  appears  to  me  both  macroscopically  and  microscopically  to  resemble 
a  porphyritic  diorite  in  all  respects.  Slide  230  is  a  highly  micaceous  variety 
of  the  same  rock. 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  231,  specimen  No.  22,717.    Gross-spur 
below  graveyard,  Virginia  City. 

Macroscopically  this  is  a  greenish-gray,  andesitic-looking  rock,  with 
impellucid  feldspars  and  brilliant  hornblendes.  Under  the  microscope  the 
slide  shows  a  considerable  number  of  hornblendes  and  some  augites.  The 
hornblende  is  of  the  greenish-brown  tint  common  among  the  andesites,  but 
brown  enters  very  largely  into  the  color.  It  is  not  fibrous,  but  decom- 
position into  chlorite  has  set  in  along  the  cleavages,  and,  in  the  longitudinal 
sections,  the  cleavage  prisms  separated  by  chlorite  might  possibly  be  taken 
for  coarse  fibers.  It  is  a  peculiarity  of  this  rock  that  the  iron  ore  has  been 
attacked  more  energetically  than  the  bisilicates.  Many  of  the  smallest 
grains  of  the  ore,  which  is  probably  magnetite,  may  be  seen  throughout  the 
sHde,  converted  into  a  slightly  diaphanous,  whitish  substance,  which  in  so 
far  resembles  leucoxene ;  but  between  crossed  Nicols  it  looks  more  Hke  cal- 


UTAH  PEOPYLITES.  143 

cite,  and  it  strikes  me  as  possibly  iron  carbonate.  The  quartz  is  indis- 
tinctly separated  from  the  groundmass,  and  seems  to  me  secondary.  The 
feldspars  and  the  groundmass  have  the  usual  characters  of  Washoe  horn- 
blende-andesi  tes 

Exploration  of  the  Fortieth  Parallel.     Slide  No.  232. 

By  some  mistake  this  slide  was  labeled  as  from  Berkshire  Canon, 
whereas  the  check-list  of  the  survey,  no  less  than  the  correspondence  of 
the  slide  and  specimen,  show  that  it  should  have  been  numbered  155,  and 
that  the  rock  is  the  typical  diorite  of  Mount  Davidson,  in  Washoe.  The 
descriptions  of  this  rock  and  of  the  Mount  Davidson  diorite,  like  the  slides, 
agree. 


PEOPYLITES  OF  THE  GEOGRAPHICAL  AND  GEOLOGICAL  SURVEY  OP  THE  ROCKY 

MOUNTAIN  REGION. 

Utah  propyiites. — Captain  Dutton  has  also  kindly  furnished  me  with  speci- 
mens and  slides  of  the  propyiites  mentioned  by  him  in  his  memoir  on  "The 
High  Plateaus  of  Southern  Utah." 

Geology  of  the  High  Plateaus  of  Utah.     Slide  and  specimen  No.  226.    Base  of  Mount 
Dutton,  Sevier  Plateau. 

Macroscopically  a  greenish,  granular  rock,  in  which  lath-like  feldspars 
are  separated  out  in  a  groundmass  of  a  somewhat  waxy  luster.  Under  the 
microscope  it  is  seen  that  the  rock  is  greatly  decomposed,  but  also  that  in 
a  fresh  state  it  consisted  of  plagioclase  and  augite,  with  an  iron  ore,  quartz, 
and  apatite  as  subordinate  constituents.  A  few  of  the  augites  are  fresh,  and 
are  in  every  way  characteristic,  but  most  of  them  have  been  converted  into 
chlorite,  and  the  slide  contains  a  large  number  of  excellent  pseudomorphs  of 
this  character.  The  chlorite  appears  to  be  precisely  the  same  as  that  to  which 
reference  has  been  made  so  often  in  the  foregoing  pages.  It  is  fibrous,  ex- 
tinguishes light  parallel  to  the  long  axis  of  the  microlites,  dichroizes  strongly, 
and  gives  an  aggregate  polarization  or  a  spherolitic  cross,  according  to  the 
arrangement  of  the  microlites.     In  places  epidote  may  be  seen  forming  at 


144  GEOLOGY  OP  THE  COMSTOOK  LODE. 

the  expense  of  the  chlorite.  At  least  a  part  of  the  quartz  is  primitive.  It 
contains  fluid  inclusions,  gas-pores,  and  possibly  also  glass-inclusions.  The 
larger  feldspars  are  well  developed,  very  closely  striated,  and  appear  to  give 
angles  of  extinction  of  about  18°  30'  in  orthopinacoidal  section.  The 
smaller  feldspars  are  granitoid  rather  than  microlitic  in  their  development, 
and  so  much  obscured  by  decomposition-products  as  to  make  it  uncertain 
whether  they  also  are  referable  to  oligoclase.  The  iron  ore  is  probably 
titanic,  and  is  accompanied  by  both  leucoxene  and  ferric  oxide.  The  slide 
contains  much  apatite,  a  large  part  of  it  in  unusually  long  microlites,  and  a 
little  sphene.     The  rock  appears  to  me  to  be  a  decomposed  diabase. 

Geology  of  the  High  Plateaus  of  Utah.    Slide  and  specimen  No.  274.    Gate  of  Mun- 
roe,  Sevier  Plateau. 

The  general  character  of  this  rock,  both  macroscopically  and  micro- 
scopically, is  almost  identical  with  that  last  described,  but  the  bisilicates 
have  been  entirely  decomposed,  and  the  chlorite  is  much  disseminated ; 
there  is  a  strong  probability,  however,  that  the  oi'iginal  mineral  was  augite. 
The  feldspar  best  answers  in  its  optical  characters  to  labradorite.  It  con- 
tains fluid  inclusions.  The  apatites  are  extraordinarily  large  and  abundant, 
and,  strange  to  say,  contain  numerous  fluid  inclusions. 


DESCRIPTION    OF    IliliUSTRATIONS. 

In  order  that  any  object  in  a  thin  section,  to  whicli  special  reference 
is  made,  may  be  readily  found  again  by  one  studying  the  collection,  thus 
saving  the  time  and  patience  of  the  student  and  leaving  no  room  for  doubt 


Fig.  2. — Orientation  of  Slides. 

as  to  the  exact  spot  under  discussion,  the  following  method  of  determining 
the  locality  of  such  an  object  has  been  employed  and  is  recommended  for 
general  use:  Mark  on  the  stage  of  the  microscope  two  radii  at  right  angles 
and  graduate  them  in  millimeters,  beginning  from  the  center  of  the  stage, 
numbering  them  as  in  the  figure.     Place  the  glass  bearing  the  rock  section 

10  C  L  145 


146  GEOLOGY  OF  THE  COMSTOCK  LODE. 

on  the  stage,  so  that  when  the  spot  to  be  located  is  under  the  cross-wires 
the  upper  left-hand  corner  of  the  object  glass  shall  be  within  the  quadrant 
between  the  radii,  and  the  sides  of  the  object  glass  shall  be  parallel  to  the 
same,  as  represented  in  the  figure.  The  distances,  then,  of  the  spot  in 
question  from  the  sides  crossing  the  radii  are  read  from  the  scales,  and  rep- 
resent the  rectangular  coordinates  of  that  spot  referred  to  the  upper  left- 
hand  corner  of  the  object  glass  as  an  origin,  and  are  recoi'ded  thus:  293""'; 
the  number  of  the  thin  section  being  given,  and  the  coordinates  being  placed 
after  it,  with  the  vertical  coordinate  preceding  the  horizontal.  The  process 
of  finding  any  spot  the  coordinates  of  which  are  given  in  this  manner  needs 
no  further  explanation. 

FiGr.  1.  Slide  252''^^  Brown  hornblende  passing  into  chlorite.  The  small  stippled 
white  mass  at  the  right  of  the  cut  is  secondary  quartz.  The  rock  is  a 
porphyritic  diorite.  Sierra  Nevada  mine,  1,450-foot  level;  north  drift, 
289  feet.    Magnified  170  diameters. 

Fig.  2.  Slide  326"'^^.  Brown  hornblende  passing  into  chlorite,  which  is  represented 
as  gray.  The  white  patches  are  quartz  and  calcite.  The  rock  is  earlier 
hornblende-andesite  from  the  Sutro  Tunnel,  17,100  feet  from  entrance. 
Magnified  70  diameters. 

Fig.  3.  Slide  464^i'^.  Greenish-brown  hornblende,  in  longitudinal  section.  The  cen- 
tral portion  of  the  veins  is  chlorite,  between  which  and  the  solid  horn- 
blende the  space  is  occupied  by  quartz.  The  rock  is  older  hornblende- 
andesite  from  croppings  1,200  feet  northwest  of  the  Geiger  Grade  toll- 
house.    Magnified  45  diameters. 

Fig.  4.  Slide  Sl"-^'.  Pseudomorph  of  chlorite  after  augite.  The  white  intrusive  mass 
is  feldspar;  decomposition  has  gone  on  from  the  surfaces  and  cracks,  pro- 
ducing a  green  slightly  dichroitic  chlorite,  which  remains  nearly  black 
between  crossed  Nicols.  The  fragments  have  also  decomposed  from  their 
centers  into  a  greenish-brown,  very  fibrous,  strongly  dichoritic  chlorite. 
The  rock  is  augite-andesite  from  the  Sutro  Tunnel,  10,055  feet  from  en- 
trance.   Magnified  95  diameters. 

Fig.  5.  Slide  465'^-^^.  The  outline  is  that  of  a  cross-section  of  augite.  The  smooth 
gray  tint  represents  a  felted  mass  of  chlorite,  composed  of  excessively 
fine  fibers.  The  coarsely  granular  mineral  is  epidote,  which  can  be  seen 
sending  denticular  crystals  into  the  chlorite.  In  the  upper  part  of  the  cut 
epidote  has  begun  to  develop  from  a  second  center.  The  rock  is  augire- 
andesite  from  Crown  Point  Eavine.  Another  part  of  this  slide  is  repre- 
sented in  Fig.  31.     Magnified  65  diameters. 


DESCRIPTION  OP  ILLUSTRATIONS.  147 

Fig.  0.  Slide  194"'".  Pseudomorpli  of  chlorite  after  hornblende.  Granular  epidote 
Is  developing-  from  five  distinct  centers  in  the  chlorite.  The  chlorite  close 
to  the  left-hand  upper  edge  of  the  crystal  is  couiposed  of  fibers  perpen- 
dicular to  the  crystal  face,  and  ajipears  to  resist  the  encroachment  of  epi- 
dote. The  rock  is  a  porphyritic  diorite  from  the  McKibheri  Tunnel.  Mag- 
nified 48  diameters. 

Fig.  7.  Slide  194^'^.  A  group  of  three  hornblendes  has  been  completely  converted 
into  chlorite,  and  in  these  pseudomorphs  epidote  has  developed  from  the 
.  centers  in  gr:inular  masses  and  fagot-like  bundles.  The  growth  of  epi- 
dote needles  into  the  chlorite  (which  is  shaded  a  flat  gray)  can  be  excel- 
lently observed  at  the  right-hand  edge  of  the  cut,  and  between  the  left- 
hand  and  the  middle  crystals.  In  the  left-hand  crystal  there  are  two 
small  patches  of  secondary  quartz.  The  rock  is  porphyritic  diorite  from 
the  McKihben  Tunnel.     Magnified  40  diameters. 

Fig.  8.  Slide  233^^-'^.  Pseudomorph  of  chlorite  and  epidote,  after  mica.  The  conver- 
sion to  chlorite  probably  proceeded  from  the  cleavages,  and  the  conversion 
of  chlorite  to  epidote  has  begun  upon  the  same  lines.  The  chlorite  as 
usual  is  indicated  by  a  flat  gray  tint.  Minute  denticles  of  epidote  can 
readily  be  seen  under  high  powers,  piercing  the  fibrous  chlorite  mass. 
The  rock  is  diorite-porphyry  from  the  head  of  Ophir  Eaviue.  Magnified 
30  diameters. 

Fig.  9.  Slide  199"-^'.  Pseudomorph  of  epidote  after  hornblende.  The  epidote  appears 
to  have  crystallized  from  three  different  centers,  and  the  radial  needles 
strike  entirely  across  the  crystal.  The  rock  is  a  porphyritic  diorite  from  the 
McKibben  Tunnel,  part  of  the  same  mass  the  pseudomorphic  phenomena  of 
which  are  illustrated  in  Figs.  6  and  7,  and  distant  only  eight  feet  from  it. 
It  is  the  last  stage  of  the  conversion  shown  in  Fig.  7.  Slide  199  also 
shows  epidote  developing  in  chlorite  patches.    Magnified  50  diameters. 

Fig.  10.  Slide  l^V'^-'^^.  Pseudomorph  of  chlorite  and  quartz  after  hornblende.  The 
quartz  occupies  the  central  portion  of  the  crystal,  and  seems  to  have  been 
deposited  by  substitution  for  chlorite.  The  chlorite  border  is  fibrous, 
excessively  fine,  and,  as  usual  where  this  structure  occurs,  transmits 
scarcely  a  ray  of  light  between  crossed  Nicols.  The  approximate  uniform- 
ity of  the  chlorite  zone  suggests  that  the  resistance  offered  by  it  to  decom- 
position has  exceeded  that  of  the  chlorite  for  which  quartz  has  been  substi- 
tuted. The  very  dark  spots  in  the  quartz  are  limonite,  and  there  are  two 
small  granular  bunches  of  epidote  in  the  chlorite,  at  the  lower  left-hand 
corner  of  the  cut.  The  slide  is  from  the  same  specimen  as  Pig.  9.  Mag- 
nified 100  diameters. 

Fig.  11.  Slide  295'°-'^.  Colorless  hornblende  passing  into  a  green  variety  of  the  same 
mineral  seen  in  cross-section.  A  large  hornblende  appears  to  have  been 
divided  into  cleavage  prisms  by  chloritic  decomposition,  much  as  in  Fig.  2, 
but  with  the  additional  development  of  the  clinopinacoidal   cleavage. 


148  GEOLOGY  OF  THE  COMSTOCK  LODE. 

These  prisms  are  colorless  near  the  center,  but  green  near  the  border.  The 
figure  shows  one  of  a  vast  number  within  the  same  crystal  outline,  the 
shaded  portion  representing  green.  No  change  in  the  angle  of  extinction 
is  produced  by  the  alteration.  The  rock  is  metamorphic  diorite  from  the 
Amazon  mine.     Magnified  270  diameters. 

Fig.  12.  Slide  295^^-^^.  Colorless  hornblende  passing  into  a  green  variety  of  the  same 
mineral,  longitudinal  section.  No  longitudinal  section  so  perfect  as  the 
cross-section  shown  in  Fig.  11  has  been  met  with.  Many,  however,  like 
that  portrayed  in  Fig.  12,  show  colorless  fibers  encroached  upon  by  the 
green  mineral.  This  section  also  contains  a  little  chlorite,  shaded  a 
deeper  tint  than  the  remainder  of  the  section.  The  rock  is  metamorphic 
diorite  from  the  Amazon  mine.     Magnified  40  diameters. 

PLATE    III. 

Pig.  13.  Slide  20'*-25.  Zonal  feldspar.  The  kernel  and  the  outer  zone  extinguish  light 
when  the  principal  plane  of  the  Nicols  is  inclined  at  an  angle  of  about 
14°  to  the  twinning  plane,  and  the  fine  reversed  lamellaj  are  blackest 
when  the  angle  measures  about  14°  in  the  opposite  direction.  The  inter- 
mediate zone  extinguishes  at  an  angle  of  5°  in  the  same  sense  as  the 
other  zones.  Just  within  the  outer  zone  is  a  belt  of  nearly  opaque  inclu- 
sions which  connects  with  the  grouudmass  of  the  rock  at  the  top  of  the 
figure.  The  rock  is  hornblende-andesite  from  the  quarry  1,000  feet  west 
of  the  Yellow  Jacket  east  shaft.     Magnified  50  diameters. 

Fig.  14.  Fortieth  Parallel  collection,  slide  284'^^.  Feldspar  with  rectangular  glass 
kernel.  The  two  halves  of  this  crystal  extinguish  light  at  angles  of  24° 
and  26°  to  the  twinning  plane,  and  minute  twin  lamellae  are  visible  at 
the  lower  end  of  the  section.     Magnified  140  diameters. 

Pig.  15.  Slide  349"'^^.  Augite  section  showing  discontinuous  twin  lamellae.  These 
are  shaded  dark  gray.  Two  included  crystals  of  iron  ore  are  indicated 
in  black,  and  some  chloritic  patches  in  light  gray.  The  rock  is  diabase 
from  the  Sutro  Tunnel  north  branch,  50  feet  south  of  Ophir  connection. 
Magnified  40  diameters. 

Pig.  16.  Slide  428'2-'^  Augite  with  contorted  twin-lamellae,  which  are  shown  in  black. 
The  rock  is  an  augite-andesite  from  near  the  Sutro  Tunnel  air-shaft  (beyond 
the  limits  of  the  map).     Magnified  70  diameters. 

Pig.  17.  Slide  450'=*-^^  Fragment  of  brown  hornblende  with  black  border  on  the  frac- 
tured surface,  as  well  as  on  the  crystal  faces,  and  a  second  parallel  internal 
belt  of  magnetite.  The  figure  is  from  a  hornblendic  andesite  from  a  cut 
1,000  feet  east  of  the  railroad  station  at  the  Silver  City  switch.  Magni- 
fied CO  diameters. 


DESCRIPTION  OF  ILLUSTRATIONS.  149 

Fig.  18.  Slide  194"-^'.  Horseshoe-shaped  apatite  cut  so  nearly  at  right  angles  to  the 
main  axis  as  to  remain  almost  black  between  crossed  Nicols.  It  occurs 
in  a  decomposed  hornblende.  The  rock  is  dioritic  porphyry  from  the 
McKibhen  Tunnel.    Magnified  220  diameters. 

Fig.  19.  Slide  dSi^^-^".  Mass  of  ilmenite  showing  characteristic  markings,  from  an 
augite-andesite  from  Cedar  Hill  Canon.    Magnified  70  diameters. 

Fig.  20.  Slide  182"'-2''.  A  iieculiar  secretion  in  a  glassy  augite-andesite  from  the  south- 
west flank  of  Mount  Kate.  It  is  a  brownish  mass  of  pseudo-spherolitic 
structure  filled  with  black  trichites.  It  much  resembles  a  patch  of  brown 
mold.     Many  others  occur  in  Ihe  same  slide.     Magnified  45  diameters. 

Fig.  21.  Slide  4:21"'-2''.  Symmetrically  arranged  acicular  black  inclusions  found  in  the 
hornblendes  of  diorites  and  andesites.  The  illustration  is  taken  from  a 
longitudinal  section  of  brown  hornblende,  and  the  direction  of  the  cleav- 
age is  indicated  by  the  arrow.  The  rock  is  aporphyritic  diorite  from  the 
center  of  Cedar  Hill  ridge.  Fig.  26  is  from  the  same  slide.  Magnified 
600  diameters. 

Fig.  22.  Slide  210'^-^".  Secondary  fluid  inclusion  in  feldspar.  These  inclusions  are 
absent  from  the  fresh  jjortion  of  the  same  exposure.  The  rock  is  from 
the  quarry  1,000  feet  west  of  the  Yellow  Jacket  east  shaft.  Magnified 
800  diameters. 

Fig.  23.  Slide  462"'-2^  The  illustration  shows  the  edge  of  a  feldspar  above  a  portion 
of  the  groundmass  of  the  slide.  The  feldspar  contains  inclusions  of  brown 
glass,  which  are  elongated  in  the  direction  of  the  edge  of  the  crystal,  and 
seem  thus  to  indicate  a  tendency  to  zimal  structure  in  the  formation  of 
the  crystal.  The  inclusions  also  show  a  connection  with  the  present  face 
of  the  crystal,  and  are  continuous  in  a  direction  vertical  to  the  face.  Por- 
tions of  the  viscid  glass  having  become  entangled  in  the  feldspar  during 
its  growth,  the  energy  of  crystallization  seems  to  have  been  insulBcient 
to  expel  or  cut  off  the  partially  inclosed  material.  The  rock  is  a  glassy 
younger  hornblende-andesite  from  the  Geiger  Grade  2,000  feet  northwest 
of  the  toll-house.    Magnified  200  diameters. 

Fig.  24.  Slide  35F'-".  Double  glass  inclusion  in  quartz.  No  part  of  this  inclusion 
reaches  either  the  upper  or  the  lower  surface  of  the  slide,  nor  is  there 
any  trace  of  a  crack  near  it.  The  rock  is  from  the  Overman  mine,  1,142- 
foot  level.    Magnified  750  diameters. 


PLA-TB    IV. 

In  the  description  of  the  figures  on  this  plate  and  tlie  succeeding  one, 
the  position  of  the  minerals  is  given  by  their  coordinates  referred  to  the 
lower  left-hand  corner  of  each  figure,  the  ordinates  being  written  before  the 


150  GEOLOGY  OF  THE  COMSTOCK  LODE. 

abscissas.  In  seeking  a  mineral,  it  is  convenient  to  lay  a  card,  or  rectan- 
gular slip  of  paper,  on  the  illustration  with  its  edges  parallel  to  those  of 
the  figure,  but  intersecting  the  graduated  edges  of  the  latter  at  the  given 
distances.  The  corner  of  the  card  will  then  coincide  with  the  point  sought. 
This  method  is  capable  of  any  desired  degree  of  exactness  and  permits  of 
the  indefinite  multiplication  of  references. 

Tig.  25.  Slide  213'''''^.  Grauular  diorite  from  Bullion  Eavine  at  Water  Company's 
flume.     Nicols  crossed.     Magnified  30  diameters. 

Geeen,  FIBROUS  HORNBLENDE :  20-22;  27-28;  22-13. 
Labkadorite  :  12-15 ;  14—28,  and  most  of  the  unspecified  grains. 
Quartz:  8-14;  15-23;  17-18.    The  quartz  carries  fluid  inclusions, 

some  of  which  show  active  bubbles. 
Magnetite:  19-10;  25-27. 
At  19-20  epidote  is  developing  in  a  patch  of  chlorite,  but  cannot  be  well 
observed  with  crossed  Nicols  or  with  so  low  a  power. 

Fig.  26.  Slide  421"'-^'*.  Porphyritic  diointe  from  the  center  of  Cedar  Hill  ridge.  Nicols 
crossed.    Magnified  30  diameters. 

Greenish-brown  hornblende:  20-20;  20-27;  30-21;  10-22,  etc. 
A  small  feldspar  is  inclosed  in  the  large  hornblende,  and  chlorite 
in  small  quantities  is  developing  along  the  cleavages  of  the  lat- 
ter, producing  with  crossed  Nicols  the  broad  black  markings 
noticeable  in  the  drawing. 

Feldspars  :  The  porphyritic  feldspars  in  this  slide,  as  at  10-18,  ap- 
pear to  be  labradorite.  Some  of  the  microlites  give  oligoclase 
angles  of  extinction.  The  greater  part  of  the  small  feldspars 
are  granular. 

Magnetite:  8-24;  20-23,  and  many  grains  too  small  to  appear  indi- 
vidually on  this  scale.  The  apatites  are  also  too  minute  to  be 
shown. 

Epidote  developing  out  of  chlorite  occurs  at  18-5,  but  requires  a 
higher  power  and  different  light  for  study. 

Fig.  27.  Slide  354'"-'''.  Quartz-porphyry  1,000  feet  southwest  of  Laicsoii\s  Tunnel. 
Nicols  at  45°.     Magnified  30  diameters. 

Orthoolase:  22-5;  20-25;  26-23;  17-15;  15-10. 

Quartz:  25-10;  15-25.    The  quartz  contains  bays  of  groundmass 

and  numerous  fluid  inclusions  with  moving  bubbles. 
Mica:  15-20;  5-24.    The  mica  is  wholly  decomposed  and  replaced 
by  limonite  and  other  secondary  products. 


i;H:iii,oi;irAi,  ,sfi;vKY, 


(;>.■()[. OGY  OF  THt:   COMSTOfK  LOUI-:  fi-i      PJ,,\TK    II 


•  liiliuK  Kim-tr^.lith 


II. .s.  (;K()i,()(;irAi.  sir[-;vi':Y. 


GEOLOWOF  TFFE   COAfSTOCK  LODE  &<■    Pr.ATK  Wl 


Fi^.14-. 


Tig. 15. 


Fig,  16 


mg.i8. 


Fig.  20. 


Figig. 


Kg.  21. 


¥i§.  22. 


Fig.  23. 


'S^m^^srS^''^  ^^*  ,i:i^^"^|3«5 


Fig.?. 


.TuliunBimXriiIiih. 


U.  S.  GEOLOGICAL  S[ITi'\'EY 


GEOLOGTOF  THE   COlfSTOrK  LODE  &r  FL.ini. 


FIG^25.  GRANULAR  DIORITE 


FIG,  26.  PORPHYRITIC  DIORITE 


FIG.  27:  QUARTZ  PORPHYRY 


FIG.  26:  EARLIER  DLA£ASE 


■  loJii:.-.  BiraiSCaliUi 


i 


U.S.  GEOLor.irAl,  SITJVKY. 


RKOLnnYflF  THE   CDMSTOCK  r.niJE  H-r.  pr.  V. 


FIG  29.  LATER  DIABASE  (BLACK DIKE) 


FIG. 30,  EARLIER  HORNBLENDE-ANDESITE 


FIG. 3]  :  DECOMPOSED  AUGITE-ANDESITE 


FIG.  32 :  LATER  HORNBI.£NDE-ANDESITE 


.luluuiIt>en.SI'(ilitli 


DESCRIPTION  OP  ILLUSTRATIONS.  151 

Pig.  28.  Slide  34:9^"-^^.    Earlier  diabase,  Sutro  Tunnel,  north  branch,  .50  feet  south  of 
Ophir  connection.    Nicols  crossed.     Magnilied  30  diameters. 

Labbadokite:  27-13;  27-23;  23-15;  19-27,  and  most  of  the  grains 

constituting  the  groundmass. 
AtjGITE:  11-27;  5-20;  21-33;  21-12. 
Ubalite:  22-8;  7-20.    The  augite  at  21-12  is  partly  converted  to 

uralite. 
Magnetite  or  ilmenite:  9-20;  10-12;  16-7,  etc. 


Pig.  29.  Slide  466'*-2='.     Later  diabase  ("black  dike").     Chollar  mine,  1,900-foot  level. 
Nicols  at  45°.    Magnified  120  diameters. 
Labradorite:  12-18,  etc. 

Augite:  25-26;  14-7;  14-9;  16-20;  25-18,  etc.     The  augite  is  all 
more  or  less  obscured  by  a  smoky-brown  decomposition  product, 
probably  limonite. 
Magnetite:  6-22;  16-18;  23-7,  etc. 

Pig.  30.  Slide  228"-".     Earlier  horubleude-audesite.    Knoll  just  northeast  of  Combi- 
nation Shaft.    Nicols  at  45°.    Magnified  23  diameters. 
Hornblende:  27-23. 
Labradorite:  22-10;  20-17;  17-25. 
Magnetite  and  ilmenite  :  All  the  black  spots. 
Chlorite:  10-29. 

Fig.  31.  Slide  465i^*^^.    Decomposed  augite-andesite  from  Crown  Point  Ravine.     No 
polarizer  was  used,  the  sky  light  happening  to  be  sufficiently  polarized 
to  develope  the  lamellte  of  the  feldspar.    Magnified  20  diameters. 
Labradorite:  25-25;  11-11. 

Atjgite:  19-5;  pseudomorphs  of  chlorite  after  augite,  7-19;  17-32. 
In  the  first  of  these,  epidote  is  developing  as  in  Pig.  5,  which  is 
also  from  this  slide. 
Epidote:  8-19;  18-10. 
The  mass  at  25-7  is  chlorite,  calcite,  epidote,  and  oxides.    The  black  spots 
in  the  groundmass  are  magnetite. 

Pig.  32.  Slide  473^°-^^.    Later  hornblende-andesite.    Quarry  2,000  feet  northeast  of 
Sutro  Shaft  III.    Nicols  at  45°.    Magnified  35  diameters. 
Hornblende:  19-18;  27-13;  23-3;  13-21;  14-25,  etc. 
Mica  :  19-9 ;  15-30.    The  last  is  almost  wholly  represented  by  mag- 
netite, leaving  only  here  and  there  a  particle  of  the  original  ma- 
terial. 
The  one  large  feldspar  and  all  the  microlites  appear  to  be  labradorite. 
The  magnetite  grains  are  readily  recognizable. 


152 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  2. — Silica  determinations. 
Dr.  G.  E.  Moore,  at  my  request,  made  the  following  determinations: 

Porphyritic  diorite,  from  the  head  of  Ophir  Eavine,  much  decom- 
posed, contains    - 58.56  per  cent.  SiOj 

Earlier  diabase,  Sutro  Tunnel,  19,100  feet  from  entrance,  highly 

decomposed,  contains 59.26  per  cent.  SiO^ 

Later  diabase,  Belcher  1,145,  very  fresh,  contains  49.79  per  cent.  SiOj 

Table  3. — Analysis  of  Water  from  the  GOO-foot  level  of  the  Savage  mine,  by  Professor  S. 

W.  Johnson,  of  Yale  College.^ 

One  liter  contained — 

Grammes. 

Silica 0305 

Alumina  and  ferric  oxide -  -   0009 

Chloride  of  sodium 0021 

Sulphate  of  lime    5044 

Sulphate  of  magnesia 0308 

Carbonate  of  potash  0148 

Carbonate  of  soda 1297 

Carbonate  of  magnesia 5012 

.7644 


Table  4. — Qualitative  determination  of  Gomstock  mine-waters.^ 
By  Eugene  S.  Bristol. 


1-5  O 

a  . 

it 

n  > 

o 

Hi 

3l 

111 

S  <o 

S  '^ 

£=" 
o  o 

^a  . 

H 

Dale  &  Norcross, 
west  drift   930- 
foot  level. 

Savage,    5th    sta- 
tion, north  drift. 

Ophir,  bottom  of 

new  shaft. 

Solid  cont«nto,  grammes^ 
Bases 

0. 0553 

Lime 

Magnesia  . . , 

0.3271 

Lime 

Magnesia . . 

Soda 

0.  0615 

Lime 

Magnesia . . . 

Potash  

Soda 

0.  0924 

Lime 

Mapiesia . . . 

0.  0784 

Lime 

Magnesia . . . 

0.0660 

Lime 

Magnesia . . 

Potash 

Soda 

0.080 
Lime. 
Magnesia- 
Soda. 

Soda 

Soda 

Soda 

Carbonic 

Sulphuric. 
Phosphoric  . 

Carbonic 

Carbonic  — 

Sulphuric. 

Carbonic 

Sulphuric. . 
Phosphoric  - 

Carbonic 

Carbonic. 

Sulphuric- . 

Sulphuric  .. 

Sulphuric. . 
PhoBphoric 

Sulphuric. 

Chlorine.    ] 

1 

Silicic 

Silicic  (trace) 

1 

lExploratioD  of  the  Fortieth  Parallel,  Vol.  in.,  p.  87.         2Esploration  of  the  Fortieth  Parallel,  Vol.  ni.,  p. 

3  In  100  cubic  centimeters  of  water. 


Table  l — Chemical  analyses. 

[From  the  publications  of  the  Exploration  of  the  Fortieth  Parallel,  excepting  those  by  Dr.  G.  E.  Moore.] 


Detennination. 


Diorite 

Do 

Mica-(liorite 

Porphyritic  diorite 

Metamorphic  diorite 

£arlier  diabase 

Quartz-porphyry  ("quartz-propylite")  — 

Quartz-porphyry  ("dacite") 

Horublende-andesite  ("propylite") 

Homblende-andesite 

Augite-audesite  ("homblende-andesite")  . 

Do  

Later  homblende-andesite  ("trachyte")  - . 

Do 

Do  

f    ("propylite") 

"Propylite"  horse 

Clay 

Do 


Do. 
Do. 


Locality. 


Eldorado  outcrop,  Mount  Davidson 

do 

800'  E.  of  Waller  Defeat  shaft,  point  5,621  <D.  5)  . 

CenterofCedarHillKidge(D.  2) 

Amazon  mine  (D.  7) 

Main  Sutro  Tunnel,  hanging  wall  of  Lode 

Hill  west  of  American  Flat,  'Waehoe 

Hills  above  American  City,  'Washoe 

Cross-spur,  below  graveyard,  "Washoe 

First  Hill  north  of  Gold  Hill  Peak,  Washoe 

Kidge  northeast  of  American  Flat,  "Washoe 

Silver  Terrace,  "Washoe 

Cross-Spur  quarry,  Washoe 

Mount  Rose,  Washoe 

do 

Washoe  ("Virginia  City)  

Yellow  Jacket,  830-foot  level 

Tellow  Jacket  6&at  clay 

Cftoliar  west  clay 

Hale  liNorcroBS  east  clay 

5at!a/7e  second  station 


Analyst. 


R.  W.  Woodward . 

...do 

Gideon  E.  Moore. . 

...do  

...do 

...do 

W.G.Mixter 

C.  Coiincler 

W.  G.  Mlxter 

W.Xonnann 

W.G.Mixter 

...do 

R.  W.  Woodward . 

...do 

...do 

W.  G.  Mixter 

...do 

S.  W.  Johnson 


W.  G.  Mixter  . 


-.-do; 

S.  W.  Johnson  . 


SiO, 


56.71 

30.24 

56.58 

30.17 

65.68 
35.01 

58.55 
3121 

46.65 
21.86 

56.40 
30.06 

68.44 
3650 


60.82 

32.43 

61.12 

32.  .W 

68.33 
31.10 

59.22 

31.58 

63.13 
33.67 


63.13 
33.67 

58.66 
31.28 

80.27 
60.02 

69.71 

65.69 

39.52 


TiOj 


0.98 
0.39 

0.83 
0.33 

1.02 
1.41 

1.14 

0l4« 


AljOs 


18.36 
8.55 


18.20 
8.18 


15.87 
7.41 


15.48 
7.23 


15.99 
7.47 

14.86 
6.92 

17.9 
8.34 

17.54 
8.17 


18.17 
8.46 

18.20 
8.48 

16.00 
7.45 

17.81 
8.30 


17.90 
a34 

0.39 
12.15 

17.59 

15.39 

15.97 


FooOs 


1.78 
0.53 


3.93 
1.18 


4.82 
1.45 


3.26 
0.98 


4.34 
1.30 

3.42 
1.02 

3.22 
0.96 


2.17 
4.38 

5.04 

2.11 


FeO 


0.45 
1.43 

6.30 
1.40 

1.25 

0.28 

2.07 
0.46 

6.99 
1.33 

3.82 


4.1 
0.91 


6.03 
1.34 

6.69 
1.48 

1.52 
0.33 

0.83 
0.18 

0.83 
■    0.18 

4.11 

0.91 


Trace 
0.11 

0.02 
0.10 


0.12 
0.03 


Trace 


CaO 


6.11 
1.74 


3.60 
1.00 

6.44 
1.84 


LOO 
0.54 


5.65 
1.61 


6.19 
1.77 

5.51 

1.57 

4.45 
1.27 

5.12 
1.46 

6.15 
1.47 

6.87 
1.67 

0.54 
6.00 

0.73 


3.92 
1.57 


1.79 
0.71 

3.60 
1.42 


3.64 
1.40 


1.3 

0.52 

1.76 
0.70 

0.61 
0.24 

2.40 
0.96 

2.90 
1.16 


2.07 
0.63 


2.06 

0,82 


0.81 

Trace 


4.41 
2.85 
3.40 


Na,0 


3.52 
0.91 

3.58 
0.92 

3.20 
0.83 

3.99 
1.03 

3.46 


3.22 
0.83 


2.0 
0.51 


3.71 
0.96 


3.20 
0.82 

3.31 
0.85 

3.87 
1.00 

4.27 
1.10 

4.44 
1.14 

2.07 
0.53 

L94 
0.45 

LOl 

2.36 


K.iO 


2.38 
0.40 

2.41 
0.41 

3.37 
0.57 

L69 
0.29 

2.02 
0.U 

L91 

0.32 


3.6 

0.61 


L41 
0.24 


3.52 
0.60 


1.30 
0.24 

2.65 
0.45 

2.26 
0.38 

2.22 
0.37 

3.10 
0.54 

2.19 
L23 

3.08 


LijO 


Trace 
Trace 


COj 


Otlier  components. 


P205. 


.0.23 
0.13 

.0.30 
0.11 

.0.44 

0.25 


Pyrite . 


Pyritel.84;  Pj  Oj 
0.34. 

Pyrito3.58i  PjOs 

trace. 
Pyiito2.84!  PjOs 
.  trace. 
Pyrite  9. 18;  PjOt 

trace. 


Ignition. 


L94 
L96 
3.10 
3.62 
2.44 
2.47 
2.26 
2.1 
2.31 
4.35 
0.70 
.  2.80 
2.00 
0.88 
0.95 
6.53 
L83 

8.09  HjO 
4.19 
2.80 

9.95  HjO 


09  39 
98.85 
100.75 
100. 61 
100.24 
99.78 

100.  50 
101.9 
100. 39« 

101.  03 
99.954 

100.  02 
100. 03 
99.96 
99  54 
100.  36 
100.  02 
99.07 


100. 24 


100.  34 
101. 00 


Specific 

gravity. 


2.65 
2.71 
2. 9582 
2. 7972 
2. 63, 2. 67 


2. 72, 2. 76 

2.6 

2.4, 2. 5, 2. 6 

2.5,2.4 


Oxygen  ratio  of— 


3.07 
2.23 


4.71 

3.51 

5.39 
3.06 

5.40 
4.06 


4.46 
3.55 


6.92 
a  19 

a34 
9.70 


&46 
10.47 


10.71 
&75 


8.34 
9.71 


SiO, 


32.43 
32.43 

32.59 
32.59 

31.10 
31.10 

31.53 
31.58 


31.28 
31.28 


o 


0.S73* 
0.285' 

0.307^ 
0.319^ 

0.397^ 
0.415* 

0.331' 


0.446' 

0.46?' 


aiog* 

0.423' 


'  Including  titanic  and  phosphoric  acida.  'Supposing  all  the  iron  present  as  ferrous  oxide. 

tGeol.  Comstock  Lode,  Vol.  III.] 


*  Supposing  all  the  iron  present  as  feme  oxide- 


'  Those  totals  do  not  agree  with  the  items,  no  doubt  in  consequence  of  misprints ;  the  oxygen-contents  of  each  constituent,  however,  cojrcspouds 
to  the  percentage  of  the  oxide  given,  and  the  errors  therefore  probably  occur  in  the  statements  of  the  carbonic  ..c.d  or  of  the  loss  by  igmfon. 


ANALYSES. 


153 


Table  5. — Analyses  of  Gomstock  ores. 


\ 


Califomiamine. 

California  mine.     Ophir  mine. 

i 

Yellow  Jaf'ket   TeUow  Jacket 
mine.                   mine. 

67.5 
8.75 
1.30 
2.25 
1.75 
.0!9 

12.85 
6.75 

65.  783 

11.35 

1.31 

2.28 

1.76 

.57 

11.  307 

6.145 

63. 38 
7.919 
1.596 
5.403 
2.786 
.059 
14.  455 
4.151 
.087 

98.  310 
.693 

96.  560 
.160 

Copper  

.575 
.150 
.005 

2.800 
.050 
.001 

Silver  

Gold 

Lead 

.25 

.267 

.429 

100.00 

100.00 

99.  896 

100.  00 

100.  00 

London. 

Swansea. 

G.  Attwood. 

W.  P.  Eickard. 

W.F.Kickard. 

\ 


Table  6. — Analyses  of  Gomstock  ores? 


Savage. 

Kentnck. 

Silica     

83.95 
1.95 
1.25 

.64 
2.82 

.85 
1.75 

.30 

.36 
1.08 

.02 
1.80 
1.28 
2.33 

91.49 
.83 
1.13 

1.37 
1.42 

.13 

.41 

.02 

.12 

.0017 

.98 
1.05 

.59 

Alumina 

Protoxide  of  manganese 

Magnesia 

Lime 

Gold 

Bisulphide  of  iron 

Potash  and  soda 

Water 

100.38          i           90.48        1 

•W.G.Mixter. 

A.  Hague. 

Table  7. — Feldspars  of  the  Yovnger  Hornblende-andesite,  from  Mount  Rose^  Slide  474.' 

Grammes. 

Weight  of  rock  treated  by  Thoulet's  uiethod 11 

Weight  of  material  of  specific  gravity  above  2.75 1.7 

Weight  of  material  of  specific  gravity  between  2.75  and  2.70 1. 8 

Weight  of  material  of  specific  gravity  between  2.70  and  2.68 1.  6 

Weight  of  material  of  specific  gravity  below  2.68  5.  8 

•  Exploration  of  the  Fortieth  Parallel,  Vol.  HI.,  page  80. 
2  Exploration  of  the  Fortieth  Parallel,  Vol.  in.,  p.  80. 
'See  page  67. 


154 


GEOLOGY  OF  THE  COMSTOOK  LODE. 


The  following  analyses  were  made  for  Dr.  G.  W.  Hawes  by  Mr.  F.  P.  Dewey: 
Feldspar  of  specific  gravity  between  2.75  and  2.70. 


Per  ceDt. 

Atomic  ratio. 

Oxygen  ratio. 

SiOa 

69.51 
23.83) 
1.545 
7.48) 
0.96  5 
1.12) 
5.355 

0.9918 
0. 2312 

0. 1576 

0.0981 

10.11 
2.46 

1.606 

1. 

31.738 
11.567 

I  3.  992 

7.95 
2.89 

1. 

AliOi 

Fe,0. 

CaO  ..      -  . 

MgO 

K2O 

NaaO 

99.79 

Feldspar  of  specific  gravity  between  2.70  and  2.68, 


1  Per  cent. 

Atomic  ratio. 

Oxygen  ratio. 

SiOj 

AUOs 

Fe,0. 

CaO 

MgO 

KaO 

FasO 

62.29 
20.  74  ) 
2.19  5 
7.01  ) 
0.65  5 
1.22) 
5.25  5 

1. 0382 
0. 2150 

0. 14]  4 

0.0975 

10.65 

2.20 

1.45 

33.22 
10.32 

I     3.82 

8.69 
2.96 

1. 

99.35 

Table  8. — Assays  of  Gomstock  rocks  J 
By  J.  S.  Curtis. 


Rocli. 


Locality. 


£.■ 


|i 
■3 


Granite 

Granular  diorite 

Do : 

Do 

Do j  Most  westerly  dioi'ite  cropping  .. 

Darlf ,  fine-grained  diorite '  McEibben  tunnel 

Dark,  coarse-grained  diorite j  Bottom  of  Union  shaft,  2,625  feet 


Red  Jacket  mine 

Bullion  Ravine,  at  intersection  of  Water  Company's  flume  . 

Bullion  Ravine,  200  feet  above  flume 

Bullion  Ravine,  2,000  feet  above  flume   


Porphyri tic  diorite 

Do 

Do 

Granular  diorite 

Micaceous  diorite-porpbyry. 

Do 

Earlier  diabase 


Head  of  Ophir  Ravine,  decomposed 

Ophir,  2,500.  Union  connection,  decomposed 

Savage,  2,  lUO,  south  drift  20  feet  from  east  cross-cot  . 

Caledonia 

Overman,  1,600,  250  feet  east  of  shaft 

800  feet  east  of  Waller  Defeat  shaft 

Sutro  tunnel  at  Savage  connection,  fresh 


$0.03 
0.19 
0.04 
0.03 
0.08 
0.11 
0.15 
0.17 
0.30 
0.12 
0.05 
0.11 
0.07 
0.22 


'  For  remarks  on  these  assays  see  page  223. 


ANALYSES. 


155 


Table  8. — Assays  of  Gomstock  rocks — Continued. 


Earlier  diabase 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Do 

Later  diabase 

Do 

Black  elate , 

Metamorphic  diorite 

Quartz-porphyry  

Do 

Earlier  bomblende-andeaite 

Do 

Do 

Do 

Augite-andesite 

Later  honiblende-andesite  . 

Do 

Basalt 


Sutro  tunnel,  50  feet  north  of  junction  with  North  Lateral,  fresh 

Overman,  1,600-foot  level  at  main  winze,  somewhat  decomposed 

Sierra  Nevada,  2,500-foot  level,  end  north  drift,  somewhat  decomposed 

0.  t£  O.,  1,650, 116  feet  from  shaft,  west  drift,  somewhat  decomposed 

Sutro  tunnel,  19, 100  feet,  somewhat  decomposed 

Sutro  tunnel,  50  feet  west  of  South  Lateral,  somewhat  decomposed 

Sutro  tunnel,  Noi-th  Lateral,  1,000  feet  north  of  C.  t£  C.  connection,  much  de- 
composed   - 

Sutro  tunnel,  North  Lateral,  600  feet  north  of  O.  d:  G.  connection,  highly  de- 
composed, charged  withpynte 

Sutro  tunnel.  South  Lateral,  250  feet  north  of  Julia,  highly  decomposed, 
charged  with  pyrite , 

Sutro  tunnel,  50  feet  west  of  South  Lateral,  highly  decomposed 

Sutro  tunnel.  North  Lateral,  250  feet  north  of  G.  &  G.  connection,  highly 
decomposed,  charged  with  pyrite 

Gkollar,  1,900,  40  feet  east  of  incline,  fresh 

Julia  dump,  fresh 

Charged  with  pyrite 

Amazon  dump 

Galedonia,  1,400-foot  level,  350  feet  east  of  Caledonia  shaft 

Quarry,  1,500  feet  southwest  of  Justice 

North  Twin  Peak 

Spur  northeast  of  Combination  shaft 

1,200  feet  northwest  of  Geiger  Grade  Toll-House , 

Near  Vivian  mine  

Forman  shaft  tank,  point  6,158 

Quarry  northeast  of  Sutro  shaft  IH 

Quarry  near  Utah  mine 

1,250  feet  southeast  of  Eoux'  Kanch 


$0.20 
O.IS 
0.17 
0.07 
0.14 
0.11 


0.11 
0.11 

0.11 
0.05 
0.11 
0.14 
0.08 
0.00 
0.03 
0.00 
0.03 
0.05 
0.04 
0.04 
0.14 
0.03 
0.17 


CHAPTER    IV. 

STRUCTURAL  RESULTS  OF  FAULTING. 

Views  of  previous  observers. — Before  proceeding  to  a  description  of  the  occur- 
rence of  the  rocks  forming  the  subject  of  the  preceding  chapter,  it  seems 
necessary  to  discuss  the  faulting  action  traceable  on  and  near  the  Lode,  for  it 
has  had  an  important  share  in  determining  the  present  position  and  relations  of 
the  rocks.  As  has  been  seen  in  Chapter  IL,  Baron  von  Richthofen  regarded 
the  Lode  as  a  true  fissure,  only  following  the  contact  between  the  syenite 
(diorite)  of  Mount  Davidson  and  the  east  country  rock  for  a  portion  of  its 
length  because  of  the  low  resistance  offered  by  this  contact.  He  also  insisted 
that  faulting  both  preceded  and  followed  the  deposition  of  ore.  He  does  not 
state,  I  believe,  whether  he  regarded  the  west  wall  of  the  lode  as  a  continuation 
of  the  exposed  surface  of  Mount  Davidson,  but  implies  that  it  is  not,  for  he 
speaks  of  the  course  of  the  vein  as  "somewhat"  dependent  upon  the  shape 
of  the  slope.  Mr.  King,  at  the  time  of  writing  his  memoir,  considered  the 
vein  as  lying  upon  a  continuation  of  the  slope  of  the  exposed  west  country, 
an  opinion  to  which  he  was  led  by  the  striking  resemblance  between  the 
contours  of  the  west  wall  and  those  of  Mount  Davidson.  Subsequently, 
from  an  examination  of  the  character  of  the  west  wall,  he  came  to  the  con- 
clusion' that  the  contact  between  east  and  west  country  was  itself  a  faulted 
surface.  Mr.  Church  recognized  abundant  evidence  of  faulting  action,  but 
regarded  the  contact  of  the  east  and  west  country  as  continuous  with  the 
exposed  surface. 

'  Privately  communicated  to  me. 
156 


STRUCTURAL  RESULTS  OP  FAULTING.  157 

Evidence  of  faulting. — The  evideiice  of  faulting  is  manifold.  The  irregular 
openings  of  the  vein,  the  presence  of  horses,  the  crushed  condition  of  the 
quartz  in  many  parts,  the  presence  of  slickensides  and  of  rolled  pebbles  in 
the  clays,  are  all  conclusive  on  this  point.  Both  to  the  east  and  west  of  the 
vein,  too,  the  country  rock  shows  a  rude  division  into  sheets,  and  along  the 
partings  between  the  plates  evidences  of  movement  are  perceptible,  decreas- 
ing in  amount  as  the  distance  from  the  vein  increases,  according  to  some 
law  not  directly  inferable.  All  the  evidence  points  to  a  relative  downward 
movement  of  the  hanffina:  wall. 

o         ft 

The  question  of  the  character  of  the  west  wall,  whether  it  is  a  faulted 
surface  or  a  continuation  of  a  former  exposure  of  the  east  front  of  Mount 
Davidson,  is  not  to  be  settled  by  mere  inspection.  A  cross-section,  to  scale, 
taken  from  Mr.  King's  maps,  shows  immediately  that  while  the  dip  of  the 
lode  is  45°  or  more,  the  maximum  slope  of  Mount  Davidson  is  about  30°. 
This  fact,  taken  in  connection  with  the  character  of  the  west  wall  where 
exposed,  indicates  that  the  surface  is  a  result  of  faulting.  A  natural  surface, 
too,  sloping  for  a  long  distance,  at  an  angle  of  about  45°,  is  very  unusual. 
On  the  other  hand  the  coincidence  between  the  contours  of  the  west  wall 
and  those  of  the  exposed  surface  has  been  recognized  from  the  earliest  days 
of  mining  on  the  Lode,  and  it  seems  a  less  violent  supposition  that  the  steep 
face  of  the  mountain  passes  over  into  the  still  steeper  wall  of  the  vein,  than 
that  the  range  has  experienced  an  erosion  modifying  its  angle  15°  and  more, 
and  has  still  retained  the  details  of  its  topography  otherwise  unaltered. 

It  is  plain  that  the  elucidation  of  the  faulting  action  on  the  Comstock 
is  a  very  important  structural  problem,  and  that  it  is  most  desirable  to 
account  quantitatively  for  the  results  as  well  as  to  prove  the  existence  of  a 
notable  dislocation,  and  no  apology  is  therefore  required  for  presenting  to 
geologists  a  somewhat  detailed  discussion  of  the  principles  involved. 

Action  offriction  on  the  surfaces  of  a  single  plate. — The  most  Striking  and  widespread 
evidence  of  the  faulting  is  the  apparent  relative  movement  on  the  contact 
surfaces  between  more  or  less  regular  sheets  of  the  east  and  west  country 
rocks  for  a  long  distance  in  both  directions  from  the  Lode.  Each  sheet 
appears  to  have  risen  relatively  to  its  eastern  neighbor,  and  to  have  sunk 


158  GEOLOGY  OF  THE  COMSTOCK  LODE. 

as  compared  with  the  sheet  adjoining  it  on  the  west.  The  consideration  of 
a  sheet  or  plate  of  rock  imder  the  influence  of  friction  of  a  relatively 
opposite  character  on  its  two  faces,  therefore,  forms  the  natural  starting 
point  for  an  examination  of  the  observed  conditions. 

Friction  a  force. — What  is  callcd  frictlon^  is  a  complex  phenomenon  which 
has  never  been  satisfactorily  reduced  to  a  mathematical  expression,  and  is 
perhaps  incapable  of  such  a  reduction.  It  is  usually  regarded  as  a  mere 
resistance,  a  force  to  which  the  negative  sign  is  indissolubly  attached.  Pro- 
fessor Reuleaux^  has  insisted  upon  the  incorrectness  of  this  view  and  has 

1  It  is  generally  considered  that  the  sensible  movements,  say  of  a  rough  block  of  stone  dragged 
over  a  pavement,  are  of  the  same  character  as  those  involved  in  the  friction  of  smoother  surfaces.  On 
the  larger  scale  it  is  plain  that  projections  of  the  moving  body  will  meet  those  of  the  underlying  sur- 
face, and  exert  a  pressure  upon  them  precisely  as  in  the  case  of  the  teeth  of  gearing.  When  the 
draught  has  reached  a  certain  intensity,  and  when  the  points  of  contact  are  small  surfaces,  approxi- 
mately normal  to  the  direction  of  translation,  the  projectious  on  one  or  the  other  surface  will  give  way, 
and  heat  will  result.  If  the  areas  of  actual  contact  are  small  surfaces,  inclined  at  a  considerable  angle, 
the  moving  body  will  rise  to  surmount  them.  In  falling  again  a  portion  of  the  energy  of  position  will 
be  converted  into  heat  by  the  impact,  but  as  all  bodies  are  to  some  extent  elastic,  the  energy  of  position 
will  not  all  be  dissipated. 

If  a  block  of  granite  is  at  rest  upon  a  pavement,  it  assumes  the  lowest  possible  position,  the  max- 
imum number  of  points  of  contact  are  established  and  the  projections  on  the  two  surfaces  overlap  to 
the  greatest  possible  extent.  When  the  same  block  is  set  in  motion,  the  energy  imparted  to  it  prevents 
its  settling  into  maximum  contact. 

It  is  plain  that  the  resistance  of  the  block  will  be  greatest  at  the  moment  when  motion  begins, 
or  that  the  so-called  friction  of  rest  is  somewhat  in  excess  of  the  friction  of  motion.  It  would  also 
seem  that  the  friction  of  rest  is  merely  the  maximum  value  of  the  friction  of  motion,  and  such  is  the 
result  of  the  recent  investigations  of  Messrs.  Jenkin  &  Ewing.  The  greater  the  velocity  of  the  moving 
body  the  less  thoroughly  will  the  projections  of  the  two  surfaces  interlock ;  on  the  other  hand, 
points  which  at  a  low  velocity  would  meet  one  another  nearly  in  vertical  lines,  will  at  high  velocities 
meet  on  a  line  considerably  inclined,  and  the  horizontal  component  of  the  elastic  force  developed  by 
impact  will  act  as  a  resistance.  Morin  took  the  elasticity  of  carriage  springs  into  consideration  in  deter- 
mining the  resistance  of  a  pavement  to  the  passage  of  vehicles.  It  appears  to  me  that  it  must  also 
enter  into  the  true  expression  for  the  coefficient  of  friction.  The  excess  of  the  friction  of  rest  over  that 
of  motion  is  evidently  due  in  part  to  the  fact  that  when  at  rest  the  energy  of  position  which  must  be 
overcome  is  at  a  maximum,  while  after  motion  has  set  in  a  portion  of  this  energy  is  elastically  returned 
to  the  moving  body.  Besides  those  elements  of  friction  which  have  been  mentioned,  adhesion  also 
undoubtedly  plays  a  part,  at  least  in  the  case  of  very  smooth  surfaces. 

The  following  deductions  from  the  experiments  of  Coulomb  and  Morin  are  approximations  only : 

(1.)  Friction  is  proportional  to  the  pressure  normal  to  the  contact  of  the  rubbing  surfaces. 

(2. )  It  is  independent  of  their  extent. 

(3.)  It  is  independent  of  their  velocity. 

According  to  Rankin  the  excess  "of  friction  of  rest  over  the  friction  of  motion  is  instantly  de- 
stroyed by  a  slight  vibration."    A  vibration  of  course  develops  the  elastic  force. 

The  friction  of  hibricated  surfaces  appears  to  me  wholly  different  from  that  of  dry  ones.  A  shaft 
should  not  come  in  direct  contact  with  its  bearing,  and  the  work  done  would  seem  to  consist  in  a  very 
active  stirring  of  a  thin  layer  of  oil.  The  amount  of  this  work  will  be  dependent  on  the  adhesion  of 
the  lubricator  to  shaft  and  bearing  as  well  as  upon  the  geometrical  character  of  the  solid  surfaces. 

^The  Pneumatics  of  Machinery,  by  F.  Reuleaus,  translated  by  A.  B.  W.  Kennedy,  p.  595.  The 
translator  states  that  similar  views  are  maintained  in  Bell's  Experimental  Mechanics,  a  work  I  have  not 
met  with. 


STEUCTURAL  RESULTS  OP  FAULTING.  ]  59 

given  instances  from  which  it  appears  certain  that  friction,  like  otiier  forces, 
may  cause  or  accelerate  motion  as  well  as  retard  it.  He  does  not,  however, 
explain  how  positive  forces  result  from  friction. 

Transmission  of  energy  by  friction. — Material  surfaces  arc  distluguislied  from 
mathematical  planes  by  the  presence  of  minute  projections  and  depressions. 
If  a  material  sheet  W  is  forced  to  move  over  a  sheet  Pj,  the  projections 
interlock,  and  if  the  sheets  are  prevented  from  moving  in  the  direction  of 
the  normal  to  their  contact  plane,  the  projections  must  either  be  ground  off 
or  be  bent  and  compressed.  If  W  begins  its  motion  with  a  fixed  quantity 
of  energy,  and  if  P^  is  fixed,  the  entii-e  energy  will  ultimately  be  expended 
in  heat,  sound,  etc.,  on  the  contact.  But  if  P^  is  movable  a  portion  of  the 
energy  of  W  will  be  communicated  to  Pj,  because  the  projections  on  the 
under  surface  of  W  exert  a  pressure  on  those  presented  by  the  upper  sur- 
face of  Pi,  which  is  either  in  the  direction  of  the  motion  of  Wor  which  may 
be  resolved  into  two  pressures,  one  of  which  is  in  the  direction  of  the 
movement  and  the  other  normal  to  the  contact  plane. 

Distribution  of  energy  through  a  system  of  sheets. If  Pj  is  lH.  COntaCt  wlth   a  third   plate 

or  sheet  Pa  the  energy  received  by  Pj  will  be  expended  wholly  or  in  part 
in  overcoming  the  resistance  on  the  contact  Pj  Pg.     If  these  sheets  are  the 

earlier  members  of  a  series  of  sheets  W,  Pj,  P2,  P3, ,  of  indefinite 

number,  then  each  sheet  which  moves  will  communicate  a  certain  amount 
of  energy  to  the  next,  and  since  the  resistance  of  friction  is  proportional  to 
the  distance  through  which  it  acts,  each  sheet  which  receives  energy  from 
its  predecessor  must  move. 

The  velocities  of  moving  sheets  may  be  treated  as  uniform. SuppOSe     a     SyStOfU     of    CQUal 

sheets  of  indefinite  extent  vertically  arranged  and  terminated  at  the  top 
by  the  horizontal  plane  A  B.  Let  the  system  be  under  a  compressive  hori- 
zontal pressure.  If,  through  the  action  of  some  external  force,  W  rises 
through  a  distance  b,  it  will  communicate  a  certain  energy  to  Pj,  which  will 
in  turn  impart  energy  to  Pg,  and  so  on.  Since  the  sheets  are  in  all  respects 
alike  and  the  pressure  at  each  contact  is  the  same,  the  frictional  resistance 
or  negative  force  at  each  contact  will  also  be  the  same,  while,  as  more  or 
less  vibration  must  always  accompany  faulting,  the  friction  of  quiescence 
does  not  need  to  be  taken  into  consideration ;  but  as  energy  is  dissipated  at 


160 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


each  contact,  the  velocity  of  the  sheets  will  not  be  equal.  According  to 
Morin's  law,  however,  the  truth  of  which  will  be  assumed,  the  frictional 
resistance  is  independent  of  the  velocity.     The  sheets  will  start  and  stop 


Fig.  3. — System  of  equal,  vertical,  movable  sheets. 

at  the  same  instant,  and  there  is  no  error,  except  that  inherent  in  Morin's 
law  in  supposing  each  sheet  to  move  throughout  its  path  at  a  uniform 
velocity. 

Ratio  of  the  movements  of  sheets. Lct    tllC    tOtal    mOVemCUt    of   P„  bC    5„  and    ItS 

entire  movement  up  to  a  given  instant  be  y^.  Then,  since  the  velocities 
may  be  regarded  as  constant, 


—  —  r- —  ^0 


and 


Vn+l 


— 1 —  ^n  > 


■'n+1 


or  the  ratio  of  the  movements  of  any  two  adjoining  sheets  is  constant.  Since 
each  sheet  controls  the  movements  of  all  its  successors,  of  which  there  are 
supposed  to  be  an  infinite  number,  each  sheet  bears  the  same  relation  to 
those  which  follow  it.     If  the  first  n  sheets  were  rejected  and  P„  were  forced 


STRUCTURAL  RESULTS  OF  FAULTING.  161 

to  move  through  a  distance  b,  P„^.i  would  move  through  a  distance  h^.     Hence 

and 

or  the  movements  of  any  two  successive  sheets  are  in  the  same  constant  ratio; 

Hence,  too, 

K+c 
Locus  of  the  edges  of  sheets. — If  BC  is  takcu  38  the  a:;-axis  of  the  locus  of  the 
edges  of  the  sheets  and  TFP,  as  the  ^-axis, 

y»  _   y   _  ^dx 

Vx-Ydx     y+dy         ' 

or 

-^  z:  1  —  m^^  z=z  —  In  mdx; 

y 

whence 

yz=.Am~'',  (2) 

which  is  the  ordinary  logarithmic  curve  and  the  equation  of  the  locus  of 

the  projecting  edges  of  the  sheets.     The  locus  of  the  other  edges  found  at 

the  reentrant  angles  is 

Modification  for  case  of  a  finite  number  of  sheets. — lu   auy  natural   Or  experimental 

case  the  number  of  movable  sheets  will  necessarily  be  finite.     The  locus 

whicli  will  be  formed  if  P„  is  fixed,  can  be  obtained  by  supposing  that  after 

the  infinite  curve 

y'  =  Am~^ 

has  been  formed,  P„  and  all  its  successors  are  forced  back  to  their  original 

position  on  the  line  AB.     Each  sheet  from  W  to  P„_i  would  then  be  drawn 

down  through  a  certain  distance,  which  can  readily  be  shown  to  be  given 

by  the  equation 

y"  =z  Km'-''  —  j:m^-2". 

The  locus  actually  assumed  will  therefore  be 

y  —  y'  —  y"—Am"'  —  Awi'-^''  —  Am-^(  1  —  Hi'<^-"'). 

Comparison  of  the  two  loci. — Thc  variatiou  of  this  equation  from  the  logarith- 
11  c  L 


162  GEOLOGY  OP  THE  OOMSTOCK  LODE. 

mic  curve  is  great  when  the  number  of  movable  sheets  is  small,  but  when 
this  number  is  great  the  effect  of  y"  on  the  locus  is  imperceptible.  If,  for 
example,  m^\A  and  n  =: 25, 

fe„  —  Am-''  —  A  lA-""'-  -  0.00022^, 

which  on  ordinary  scales  would  be  scarcely  visible.  The  value  taken  for  m 
is  one  which  has  been  noted  in  experiments  to  be  described  on  a  succeeding 
page.  If  0.0001^  is  regarded  as  a  negligible  quantity,  then  the  locus  of 
the  edges  of  the  sheets  may  be  regarded  as  coincident  with  the  logarithmic 
curve  y  ■=  Amr'^  when  for  the  first  fixed  sheet  P„ 

4 


vC^ 


log  w' 


Logarithmic  distribution  of  energy. — The  force  cxcrted  at  cach  contact  of  a  system 
of  sheets  is  that  of  friction,  and  when  the  friction  is  uniform  throughout 
the  system,  only  the  distance  through  which  the  force  acts  at  each  contact 
varies  with  its  distance  from  the  first  contact.  If  on  the  contact  P„  P„+i  the 
surfaces  are  such  as  to  present  a  greater  or  smaller  number  of  opposing 
projections  per  linear  unit  than  exists  upon  other  contacts,  the  force  or  friction 
would  also  differ.  But  the  energy  received  by  P„  would  be  unaffected  by 
this  difference,  and  the  ratio  of  the  energy  expended  upon  the  contact 
Pti  -P»+i  to  that  transmitted  to  subsequent  contacts  will  depend  not  upon  the 
number  of  projections  but  upon  the  physical  (elastic)  properties  of  the 
material  of  which  the  sheets  are  composed.  By  Morin's  law  the  friction, 
and  therefore  also  this  ratio,  are  unaffected  by  the  velocity,  and  the  same 
amount  of  work  will  consequently  be  done  on  the  contact  P„  F^+x  as  if  the 
friction  were  the  same  as  on  other  contacts.  If  the  whole  energy  applied 
to  the  system  is  ^,  and  if  the  frictional  resistance  on  the  successive  con- 
tacts is/, /i, /a,  etc., 

^=/(6-ii) +/(&,- 62)+  •  ■  •  +/„(^'«-^'n+i)+  •  •  • 

The  absolute  movements  of  the  sheets  will  be  dependent  upon  the  total 
energy  and  upon  the  different  resistances,  and  so  also  will  be  the  curve  or 
broken  line  assumed  by  the  edges  of  the  sheets;  but  any  term 

/«  (^„  —  ^n+l) 


STRUCTURAL  RESULTS  OF  FAULTING.  163 

is  dependent  only  upon  E.     If  w  is  the  work  done  on  any  contact, 

and  if  L  denotes  the  work  done  on  the  first  contact,  WP^,  the  general 
equation  for  the  work  on  all  contacts  is 

or  the  distribution  of  energy  is  logarithmic  however  the  friction  may  vary, 
so  long  as  the  material  composing  the  sheets  is  the  same  throughout  the 
system,  and  supposing  friction  independent  of  velocity. 

Morin's  law  is  mei'ely  an  approximation,  but  should  an  exact  relation 
be  discovered  between  friction  and  velocity  it  would  be  an  easy  matter  to 
give  the  variation  of  the  friction  its  proper  weight  in  the  equation  for  a 
faulted  surface. 

Locus  of  edges  of  sheets  when  the  friction  varies  regularly. CaSeS     may     rCadily     aHsO      lu 

which  the  friction  varies  regularly  from  contact  to  contact,  as  would  hap- 
pen for  example  in  a  system  of  sheets  between  which  the  pressure  was  pro- 
duced by  the  weight  of  the  sheets  themselves.  Suppose  the  case  of  friction 
increasing  from  /  at  the  contact  TF  P  by  a  small  increment  ft.  Then  for 
any  distance  x  from  the  origin,  the  frictional  resistance  will  be/(l+  ^0- 
If  dx  is  the  thickness  of  a  sheet,  the  relative  motion  at  x  will  be  dy  and  the 
work  done/  (1  +  xt)  dy.  If  the  friction  were  constant  and  equal  to/,  the 
work  done  on  the  same  contact  would  be  derivable  from  an  equation,  say 

y-^z=.Am~^, 
and  would  amount  to 

fdy-iz^ — -fA  In  m  nr^  dx ; 

and  since  it  has  been  shown  that  the  work  on  any  contact  is  independent 

of  the  frictional  resistance, 

f{l-\-xt)dyz:z—fA  In  mm~^dx; 
or 

^  ,        pmr^dx 

y=-AlnmJj~^, 

which  is  not  integrable  when  m  >  1. 

Approximate  equation. — If  thc  pressurc  Is  produced  by  the  weight  of  the 
sheets  and  if  these  are  numerous,  t  is  a  very  small  quantity  and  its  square 


164  GEOLOGY  OF  THE  COMSTOCK  LODE. 

may  sometimes  be  to  the  senses  a  vanishing  quantity.     When  this  is  the 
case  the  equation 


\-\-xt 

sensibly  represents  the  locus.     For  the  value  of  iv  may  be  written 

Jyf{\+xt)=f(h-h,)m-^ 


or 


^     l4-xt  \l4-xt     \+xty 


'l+xt  \l-i-xt     I+rct 

while  the  approximate  equation  gives 


^     \l+xt     .  .     xt  J 


and  since 

the  two  equations  give  the  same  results,  if  ^2  is  inappreciable. 

It  has  already  been  pointed  out  that,  since  the  distribution  of  energy 
is  logarithmic,  the  sum  of  the  relative  movements  is  dependent  on  the  vari- 
ation of  the  friction.  If  therefore  the  friction  is  a  minimum  at  the  contact 
W  Pi,  a  greater  amount  of  energy  will  be  required  to  move  W  through  a 
distance  A  than  if  the  friction  were  constant.  The  total  energy  required 
will  be  the  same  as  it  would  be  if  each  relative  movement  took  place  by 
itself  Assuming  the  approximate  equation  deduced  for  this  case,  it  can 
readily  be  shown  that,  if  W  moves  a  distance  A,  the  total  energy  required 
by  the  system  is 

-^^O  +  ^fTT^)- 

Since  there  is  nothing  essentially  positive  in  the  nature  of  t,  all  the 
foregoing  equations  become  applicable  to  the  case  of  a  decreasing  frictional 
resistance  by  merely  reversing  the  sign  of  t.  Landslides  might  furnish 
cases  of  this  character.  Suppose  a  mass  of  material  divided  into  sheets 
resting  on  a  hillside,  and  that  through  weakened  coherence  the  mass  de- 
scended  such  a  distance  as  miglit  be  necessary  to  do  a  work  f  A  om.  tlie 


STEUCTURAL  RESULTS  OF  FAULTING.  165 

contact  W  Pj.  This  energy  will  be  distributed  through  the  system,  and 
were  the  friction  uniform  the  resulting  curve  would  be  a  simple  logarithmic 
one.     But  as  the  friction  will  decrease  towards  the  surface,  the  locus  will 

be  approximately 

Amr" 

To  produce  this  configuration,  however,  an  energy  of  only 

is  required,  and  the  system  will  consequently  reach  it  with  a  vis  viva 

fAt2^=fA,. 
,   1 — xt 

The  system  will  continue  its  movement  till  this  energy  is  expended  and  its 
final  configuration  will  be  ^ 


y  =  {A  +  A,)^_ 


■xt' 


Experimental  verification. — If  the  various  assumptious  made  are  correct,  a 
fault  under  certain  conditions  will  result  in  a  surface,  a  vertical  section  of 
which  at  right  angles  to  the  strike  of  the  fault  will  present  a  logarithmic 
curve.  Before  proceeding  to  any  further  deductions,  it  is  evidently  desir- 
able to  test  the  correctness  of  the  postulates  experimentally.  I  have  sup- 
posed the  sheets  of  rock  of  infinite  size  as  compared  with  their  exposed 
margins,  because  on  this  supposition  the  pressure  per  unit  of  area  of  each 
parting  will  be  the  same.  If  the  plates  were  thoroughly  flexible,  and  if 
the  pressure  were  applied  on  a  limited  zone  parallel  to  the  croppings  and 
removed  by  a  distance  greater  than  h  from  either  end  of  the  plates,  then 
the  pressure  exerted  on  each  plate  would  be  the  same,  and  would  be  dis- 
tributed over  an  equal  area,  and  the  resulting  curve  would  still  answer  to 
the  general  formula  deduced.  These  conditions  we  can  approximately 
reproduce.  If  a  pile  of,  say,  one  hundred  slips  of  very  thin,  flexible  and 
uniform  paper,  eight  or  ten  inches  long,  with  sharply  cut  edges,  are  laid 
upon  a  flat  surface,  and  a  narrow  weight  of  three  or  four  pounds  is  placed 
across  them,  the  pressure  under  the  weight  may  be  considered  as  constant. 


166  GEOLOGY  OF  THE  COMSTOCK  LODE. 

In  the  experiments  I  have  made  the  weight  employed  was  about  5,000 
times  as  great  as  that  of  a  single  slip.  If  a  blunt  edge,  such  as  that  of  a  ruler, 
be  now  applied  at  right  angles  to  the  longer  dimension  of  the  slips,  close  to  the 
weight,  with  a  light  pressure,  and  be  drawn  away  from  the  weight  a  fraction 
of  an  inch,  a  slight  relative  movement  will  be  perceptible.  If  this  applica- 
tion of  energy  to  the  system  be  repeated  a  score  of  times,  the  ends  of  the 
pile  of  slips  will  be  found  to  form  a  curved  surface  instead  of  a  plane  ^ 
If  the  frictional  resistance  is  proportional  to  the  pressure,  this  curve  must 
sensibly  coincide  with  that  given  by  the  equation 


y= 


l+xt' 


for  i"— p—TT-a ,  and  will  altogether  escape  detection.      The  thinness  of  the 

paper  considerably  obscures  the  character  of  the  curve,  but  there  is  no 
error  in  principle  involved  in  plotting  it  on  the  assumption  that  the  sheets 
are  of  any  thickness  which  may  seem  best  adapted  to  bring  out  its  geo- 
metrical relations.  For  the  given  increment  the  curve  will  approximate 
pretty  nearly  to  the  simple  logarithmic  curve.  For  the  one  hundredth-con- 
tact the  latter  would  give  * 

and  the  equation  for  increasing  pressure 

_  Am-"" 
^~1+U.02' 

or 

y,=1.02  y. 

Unless  the  experiment  is  carried  on  until  the  lowest  movable  sheet  has 
traversed  a  sensible  distance,  the  original  position  of  the  edges  of  the  sheets 
marked  by  the  fixed  slip  gives  the  asymptote  of  the  original  curve.  Fig.  4 
on  the  next  page  shows  a  curve  AB  plotted  from  experiment  with  its  asymp- 
tote, and  a  logarithmic  curve  CD  of  the  form  y=iAmr-^  plotted  from  its  equa- 

'I  noticed  long  since  that  pressmen  in  printing  offices,  by  drawing  the  thnmb-nail  across  a  pile 
of  sheets,  force  each  of  the  upper  sheets  to  project  beyond  the  one  next  beneath  it,  so  that  one  sheet  at 
a  time  can  be  removed  conveniently  and  without  delay.  I  observed  that  a  regular  curve  resulted,  but 
jiresumed  that  it  was  a  conic  section.  Having  satisfied  myself  analytically  that  the  curve  produced 
by  faulting  was  log.arithmic,  this  observation  recurred  to  me  as  a  means  of  testing  my  results  experi- 
mentally. 


STRUCTUEAL  RESULTS  OF  FAULTING. 


167 


WMBk 


tion.  The  deviation  is  exceedingly  slight,  and  the  experimental  curve 
stands  almost  as  well  as  the  other  the  very  delicate  constructive  test  of  the 
equality  of  subtangents.^ 

Variations  of  the  experiment. — The  sHps  I  liave  cmploycd  are  of  a  nearly  iincal- 
endered  paper.  If  for 
one  of  them  a  highly 
glazed  slip  is  substituted 
a  comparatively  large 
relative  motion  takes 
place  on  its  surfaces,  but 
the  only  visible  effect 
which  the  introduction  of 
such  a  slip  produces  on 
the  locus  of  the  others  is 
a  dislocation  at  the  point 
where  it    is    inserted. 

There   is   in   fact   no    evi-  Fig.  4.— Calculated  and  observed  eiirves. 

dence  that  the  work  done  on  any  contact  is  altered  by  the  introduction  of 
a  contact  offering  a  smaller  frictional  resistance. 

If  the  ends  of  the  slips  at  the  beginning  of  the  experiment  occupy  an 
inclined  instead  of  a  vertical  plane,  the  result  is  a  logarithmic  curve  referred 
to  axes  inclined  at  the  same  angle  In  plotting  it  is  well  to  reject  the 
upper  three  or  four  slips,  because  these  are  principally  affected  by  inequal- 
ities in  the  application  of  pressure  and  draught. 

By  employing  a  system  of  from  three  to  ten  slips  of  heavy  writing 
paper,  using  a  thick  pad  of  blotting  paper  for  a  support,  and  applying  the 

■Such  an  experiment  forms  a  check  upon  the  theory,  but  does  not  furnish  absolute  proof  of  it, 
because  arcs  of  other  curves,  known  or  unknown,  might  be  constructed  which  would  agree  very  closely 
with  the  experimental  result.  Among  familiar  curves,  that  presenting  the  greatest  general  similarity 
to  the  logarithmic  curve  is  the  hyperbola  referred  to  in  its  asymptotes,  and  a  hifperbolic  arc  very  closely 
agreeing  with  the  experimental  curve  can  be  calculated.  But  the  experiment  gives  the  position  of  the 
asymptote  which  for  the  nearest  hyperbolic  arc  would  occupy  a  distinctly  different  position,  and  the 
supposition  that  the  curve  was  hyperbolic  would  also  lead  to  seemingly  untenable  hypotheses  as  to  the 
communication  of  energy.  All  that  can  be  claimed,  however,  strictly  speaking,  is  that  the  theory  ac- 
counts for  the  facts  within  the  limits  of  the  errors  of  observation,  and  that  no  other  equally  plausible 
explanation  of  the  facts  has  suggested  itself  to  me, 


168 


GEOLOGY  OF  THE  COMSTOGK  LODE. 


draught  with  great  care,  the  locus 

can  be  produced  on  such  a  scale  that  both  its  elements  are  sensible. 

Reduction  and  interpretation  of  the  equation. — A  few  data  as  to  the  com- 
putation and  representation  of  the  logarithmic  curve  may  be  of  use  to 
those  who  bave  to  do  with  special  cases  of  faulting,  either  technically  or 
geologically. 

Equation  referred  to  the  cropping  as  origin. In  the  form  of  the   CqUatlon   deduCed, 

yzz:Am~'',  (1) 

the  curve  is  referred  to  its  asymptote  and  the  fault  line  as  axes.  In  ascer- 
taining the  value  of  the  constants  applicable  to  any  given  surface,  however, 
it  will  be  more  convenient  to  refer  it  to  the  fault  line  and  a  line  perpendic- 
ular to  the  latter  at  the  point  where  it  reaches  the  earth.  If  the  fault  dips 
at  90°,  and  if  the  original  surface  was  level,  the  equation  will  then  be 

y=A{m-^-\)  (2) 

If  the  original  surface  was  not  horizontal,  but  formed  an  angle  5  with 
the  a;-axis,  then  retaining  the  same  axes  each  y  will  be  diminished  by  x  tan  3, 
and  the  equation  becomes 

y—A  (m-^— 1)— ictan  5;  (3) 

and  in  this  case  the  asymptote  of  the  curve  would  still  cut  the  y-axis  at  —A, 

but  would  make  an  angle  5 
with  the  ic-axis  or  would  be 
parallel  to  the  original  sur- 
face. Since  the  angle  S^ 
merely  expresses  the  rela- 
tive directions  of  the  .x-axis 
and  the  original  surface,  this 
equation  is  general,  and  ap- 
plies, whatever  may  be  the 
dip  of  the  fissure  and  what- 
FiG.5.-j,=^(m-.-i)-xtau3.  ^^,^^    ^^^    ^i^^^    been   the 

slope  of  the  original  surface.     If  f3  is  the  dip  of  the  fissure  and  d  is  the 
slope  of  the  original  surface,  Ave  also  have 

5  =  90°-/?zb<^, 


STRUCTURAL  RESULTS  OF  FAULTING.  169 

in  which  d  has  the  positive  sign  if  the  surface  sloped  in  the  same  sense 
as  the  fissure  plane,  and  the  negative  sign  if  the  dip  and  the  slope  were  in 
opposite  directions.^  This  formula  therefore  makes  it  possible  to  recon- 
struct the  original  surface,  in  so  far  as  it  is  unmodified  by  other  causes. 

Reduction  of  equation  to  simplest  form. — Equatlou  (3)  is  thc  most  couvenient  form 
for  the  calculation  of  the  constants  involved,  because  the  direction  of  the 
«/-axis,  and  commonly  also  the  position  of  origin,  can  be  directly  observed,^ 
bat  for  plotting  and  for  some  purposes  of  discussion  the  equation  can  be 
advantageously  reduced  to  another  form.     The  equation  of  the  asymptote  is 

y-\-A^x  tan  S-. 

If,  therefore,  we  refer  equation  (3)  to  the  intersection  of  the  asymptote  and 
the  «/-axis  and  adopt  the  asymptote  as  a  new  z-axis,  (3)  will  reduce  to  the 
form 

'  I  have  preferred  to  characterize  these  angles  in  this  way  rather  than  to  adopt  the  ordinary  hut 
not  universal  oonventiou  as  to  positive  and  negative  augles,  because  this  is  a  discussion  of  structural 
geology.  Tlie  mathematical  question  involved  is  simply  whether  /3  and  S  lie  in  the  same  quadrant  or  in 
adjoining  ones. 

For  similar  reasons  common  logarithms  instead  of  natural  logarithms  have  been  used  in  all  for- 
mulas, the  direct  applicability  of  which  to  natural  occurrences  renders  it  possible  that  computations  may 
be  based  upon  them. 

^In  computing  the  logarithmic  curve  which  most  nearly  applies  to  a  given  surveyed  section  line 
it  is  necessary  to  know  the  dip  of  the  iissure  and  the  position  of  three  points  on  the  surface  relatively 
to  the  rectangular  coordinates  the  origin  of  which  is  the  cropping  and  the  y-axis  the  dip-line.  The 
computation  is  greatly  simplified  by  so  selecting  the  arbitrary  values  of  x  (xi,  xi,  X3)  that  Xi  =^X2=^J3. 
The  three  equations  then  become 

j/i  =  ^{m~^'  — 1)  — x,  tan  9; 
1/1  =  A  (m~-^'  —  l)  —  '2xi  tan  5; 
y3  =  A  (»»-**'— l)  —  4xi  tan  S. 
Solving  these  equations  for  the  three  constants,  it  will  be  found  that 


log  m- 


A  =  —      ^2/1— J/'! 


tan  ^^A 


(m-="— 1)'' 
(m-»'-l)-y, 


170  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Mere  inspection  also  shows  that 

Xi-:^x  cos  5, 

and  the  equation  refeiTed  to  the  inclined  coordinates  indicated  will  be 

y=A'm-'""'^\  (4) 

By  a  proper  selection  of  a  unit  and  by  removing  the  origin  to  a  different 
point  on  the  a;-axis  according  to  well  known  rules  of  analytical  geometry,' 
this  equation  may  be  reduced  to  the  form  shown  in  Fig.  6, 

2/=lo-^  (5) 

or 

x=.  —  \ogy; 

and  the  points  on  the  curve  may  be  directly  plotted  from  a  table  of  log- 
arithms.    The  curve  evidently  cuts  the  ^/-axis  at  the  point  where  y  is  equal 

to  the  natural  unit  y,  found  as  indicated  in  the  foot-note.  If  the  equation 
were  plotted  on  rectangular  coordinates,  ^  would  also  be  the  constant  value 

'As  the  COMSTOCK  Lode  excites  a  lively  interest  in  many  localities  where  books  of  reference  are 
rare,  it  may  be  a  matter  of  convenience  to  some  of  my  readers  to  give  this  reduction  in  fnll. 
Let 

A  =  cos  3  log  m, 

or 

10''  =  m'^<""»; 

then  introducing  this  value  into  (4),  we  have 

Let  the  origin  be  transposed  ou  the  x-axis  by  a  quantity  a,  yet  to  be  determined ;  then 

;i=A  10-''<*+"'=^  !()-'"'  lU-*^. 
Now  let 

logM-logj4 
h 

The  introduction  of  this  valne  brings  the  equation  to  the  form 

%=io-'^, 

because  for  the  chosen  valne  of  a 

If,  further,  —is  taken  as  the  unit  and  x  and  y  are  each  multiplied  by  it,  we  get 
h 


STRUCTUEAL  RESULTS  OF  FAULTING. 


171 


of  the  subtangent,  and  the  curve  would  cross  the  y-a\is  at  an  angle  of  45° ; 
but  this  is  not  the  case  when  the  equation  is  interpreted  on  oblique  coor- 
dinates. 


Fig.  6.     ^  =  lO--". 
Point  of  minimum  radius  of  curvature. The  pOsitioU  of  tlie  IJ-'dXlS  of  tile  logaHth- 

mic  curve  depends  upon  the  unit  chosen.  There  is,  however,  one  fixed  point 
on  the  locus,  that  of  minimum  radius  of  curvature.  This  must  be  deduced 
from  the  general  equation  referred  to  rectangular  coordinates  (3),  and  the 
value  of  X  corresponding  to  it  is 

_log  (4  A  Inm)  —log  (V8  +  9  tan^^  — tan  5) 

log  m 

From  this  formula  the  value  of  Xo  for  all  simpler  cases  can  easily  be 
derived.     For  the  simplest  equation,  viz: 

ln2      J  1 

Xo=-~-  and«/o=— -. 
^  v2 

Spacing  of  contours. — As  tlic  topographj  of  a  country  is  usually  represented 
for  geological  purposes  by  contours,  it  would  be  interesting  to  discuss  the 
spacing  of  the  contour  lines  on  a  map  of  a  faulted  surface.  For  an  origi- 
nally level  surface  and  a  vertical  fault  we  have  immediately 

^x=]ogy—log(y+Jy) ; 

in  which  ^  a;  is  the  variable  horizontal  interval  between  contour  lines  and 


172  GEOLOGY  OF  THE  COMSTOCK  LODE. 

J  y  the  constant  vertical  difference  between  contour  planes.  But  the  equa- 
tion for  the  case  of  an  oblique  fault  is  so  complicated  as  to  be  of  no  value. 
The  ideal  map  would  be  one  in  which  the  contour  planes  were  so  close  that 

—  would  be  sensibly  equal  to  ~j-  ;  and,  indeed,  where  the  slope  is  consid- 
erable this  is  often  the  case,  but  when  the  surface-line  becomes  nearly- 
horizontal  the  difference  between  the  two  ratios  is  large. 

Angle  of  tangent  to  the  horijonai. — The  angle  which  &  tangent  to  the  curve 

y  =  \Q-' 

referred  to  inclined  coordinates  makes  with  the  horizontal  may  be  found  as 

follows,  without  going  through  a 
troublesome  transformation  of  coor- 
dinates. Let  dx  and  dy  be  the  differ- 
entials  at  the  point  of  tangency 
obtained  from  the  above  equation, 
and  dx^  and  dy^  the  differentials  for 
the  same  point  if  the  ?/-axis  were 
Fig.  7.— Explanation  of  a  faulted  surface.  vertical  and  the  ic-axis  horizontal. 
Consider  ^  as  a  positive  acute  angle  and  S  also  as  a  positive  acute  angle 
when  it  falls  in  the  same  quadrant  with  /?,  but  as  negative  when  it  falls  in 
an  adjacent  quadrant.  Let  a  be  the  angle  which  the  tangent  makes  with 
the  horizontal.     Then,  as  appears  from  the  figure, 

tan  a  —  —  -  ^, 

dx-^ 

and  the  equation  of  the  curve  referred  to  inclined  coordinates  gives 


ylnlOzz:-^^, 
dx 


and  by  a  simple  projection 


_  —  c?«/ sin  y5 -fete  sin  <5_ 
~~  —  dy  cos  /3  -\-  dx  cos  8^ 
or  by  reduction 

.  _  y\n  10  sin  yS  -f  sin  (5 

«/ln  10  cos/?  +  cos(5 


STKUCTDEAL  RESULTS  OF  FAULTING. 


173 


If  (5  is  a  minus  angle  (the  case  shown  in  the  figure)  the  curve  will  be 
horizontal  when 

—  sin  (5  —  2/  In  10  sin  /?, 


or  when 


y  = 


—  sin  S 
In  10  sin  A' 


but  if  (J  is  a  positive  angle  (falling  in  the  same  quadrant  with  ft)  the  curve 
will  have  no  horizontal  tangent. 

Fault  involving  double  curvature. — As  has  already  bccu  pointed  out,  since  gravity 
is  likely  to  be  an  insignificant  force  compared  with  other  forces  acting  on 
the  sheets  of  a  faulted  country,  it  is  a  matter  of  indiflFerence  whether  we 
regard  the  actual  motion  of  the  foot  wall  as  upward  or  that  of  the  hanging 
wall  as  downward.  If,  therefore,  contrary  to  the  assumption  thus  far  made, 
the  foot  wall  instead  of  the  hanging  wall  were  divided  into  sheets,  and  if  the 
latter  were  to  sink  relatively  to  the  former,  we  should  get  a  reversed  loga- 
rithmic curve  asymptotic  to  the  original  surface  of  the  foot  wall;  and  other 
things  remaining  equal,  its  equation  would  be 

yzz.  A{\  —  trf)  -\- x  tan  S^. 
If  the  rock  on  both  sides  of  the  fissure  is  the  same,  or  possesses  the 


Fig.  8. — Double  fault  curve. 


saine  physical  properties,  and  is  divided  into  plates  of  the  same  thickness, 
the  energy  brought  to  bear  at  the  fissure  will  be  distributed  in  both  direc- 


174  GEOLOGY  OF  THE  COMSTOCK  LODE. 

tions  on  the  contacts  between  the  plates,  and  the  cross-section  of  the  coun- 
try will  show  two  logarithmic  curves  with  a  common  tangent  at  the  origin 
in  Fig.  8.     Each  curve  can  of  course  be  reduced  to  the  form 

Case  involving  different  rocks. — If  the  fissurc  wcre  ou  a  contact  between  two 
different  rocks,  the  one  might  be  divided  into  thinner  plates  than  the  other, 
and  they  might  have  different  coefficients  of  friction.  If  the  coefficient 
being  the  same  the  thickness  of  the  plates  varied,  the  origin  would  remain 
unchanged,  but  the  curves  would  be  different.  The  curvature  depends  on 
the  throw  of  the  fault  and  on  the  number  of  partings,  and  it  can  readily  be 
shown  that  the  natural  unit  of  the  curves  formed  will  be  proportional  to 
the  thickness  of  the  sheets  of  rock.  The  two  curves  will  therefore  not 
have  a  common  tangent.  Conversely  it  is  evident  that  the  relative  thick- 
ness of  the  sheets  is  calculable  from  the  observed  curvature,  but  the  abso- 
lute thickness  of  the  one  or  the  other  is  a  matter  of  observation.  If  the 
coefficients  of  friction  are  unequal,  the  inequality  will  manifest  itself  only 
at  the  contact,  for  the  fundamental  equation  of  condition 

is  independent  of  /  so  long  as  /  is  constant.  The  curves,  however,  will  not 
be  continuous  with  one  another.  There  is  reason  to  suppose  that,  at  least 
between  similar  rocks,  the  difference  of  the  coefficients  of  friction  is  very 
small. 

Faulting  accompanied  by  formation  of  parallel  fractures. it     a     lault     takeS     place     OU     a 

fissure  in  otherwise  solid  rock,  and  if  lateral  pressure  accompanies  the  dislo- 
cation, a  great  amount  of  energy  will  be  brought  to  bear  at  the  fissure. 
If,  as  before,  the  foot  wall  is  supposed  to  rise,  the  hanging  wall  as  a  whole 
may  be  regarded  as  a  fixed  mass  either  from  its  cohesion  with  the  surround- 
ing country,  or  from  the  indefinite  amount  of  inertia  which  it  opposes  to  move- 
ment. As  has  been  shown  earlier  in  this  chapter,  friction  is  a  force  which 
produces  motion  as  well  as  destroys  it,  and  Professor  Reuleaux  is  doubt- 
less correct  in  asserting  that  motion  always  results  from  friction,  although 


STRUCTURAL  RESULTS  OF  FAULTING. 


175 


it  may  be  "only  as  small  alterations  of  form  in  the  body  acted  upon" 
Rocks  are  by  no  means  absolutely  rigid  or  absolutely  inelastic,  and  under 
the  conditions  supposed  a  strain  must  be  produced  in  the  hanging  wall. 
Sedimentary  strata,  and  especially  the  coal  measures,  furnish  innumerable 
known  examples  of  this  action,  indicated  by  the  permanent  flexure  of  the 
ends  of  the  strata  as  indicated  in  Fig.  9.  This  is  of  course  a  familiar  fact 
which  has  from  time  imme- 
morial furnished  miners  with 
a  practical  rule  for  recovering 
the  seam  beyond  a  fault. 

When  a  fault  takes  place 
in  the  comparatively  rigid 
massive  rocks  a  similar  strain 
must  also  be  produced.  Its 
effect  will  depend  upon  its  in- 
tensity and  on  the  elastic  pro- 
perties of  the  rock.  These 
latter  are  so  little  known  that 

it  is  scarcely   worth  while    to  *'i«-  9.-Fault  accompauiedby  astiain. 

investigate  the  conditions  mathematically,  but  it  is  certain  that  if  the  strain 
surpasses  a  limit  defined  by  the  cohesion  of  the  rock,  a  sheet  of  the  latter 
will  be  sheared  off  from  the  main  mass.  If  the  compression  attending  a 
fault  in  a  massive  rock  is  very  great,  and  if  the  rock  is  very  rigid,  this  action 
may  be  repeated  indefinitely,  and  either  or  both  walls  may  be  divided  into 
sheets  of  nearly  equal  thickness  and  divided  by  partings  nearly  parallel  to 
the  original  fissure.  On  the  other  hand,  if  the  stress  does  not  reach  the  ulti- 
mate cohesive  resistance  of  the  rock,  the  energy  must  be  expended  in  heat 
and  a  strain  which  will  be  permanent  or  not  as  the  rock  is  elastic  or  in- 
elastic. 

Evidence  furnished  by  observation. — In  coal  mlncs  there  is  abundaut  evidence  of 
permanent  strains  produced  by  faulting.  In  massive  rocks  a  division  into 
sheets  sometimes  accompanies  faulting,  but  it  might  be  asserted  that  the  two 
phenomena  were  unconnected.  A  very  unobtrusive  structural  action  serves, 
however,  to  establish  a  relation.    In  hilly  regions  where  the  soil  is  deep,  small 


176  GEOLOGY  OF  THE  COMSTOCK  LODE. 

landslips  are  common  during  wet  weather,  often  involving  the  movement 
of  only  a  few  square  rods  of  ground  for  a  few  feet.  The  material  in  this 
case  is  far  from  rigid,  but  on  the  other  hand  it  possesses  a  minimum  of  elas- 
ticity. I  have  examined  hundreds  of  such  slips  in  the  Contra  Costa  Hills 
of  California,  and  noted  with  surprise  the  fact  that  they  are  almost  invaria- 
bly accompained  by  a  separation  of  the  moving  mass  into  sheets  far  more 
regular  than  might  have  been  expected,  and  parallel  to  the  initial  surface  of 
motion. 

It  does  not  appear  to  me  that  the  character  of  the  curve  assumed  by 
the  edges  of  the  sheets  will  be  affected  by  the  consumption  of  energy  in- 
volved in  shearing  them  from  the  mass  of  country  rock,  for  the  work  done 
at  each  fracture  will  be  the  same  and  the  effect  will  appear  in  the  constants 
of  the  equation,  not  in  the  form  of  the  function. 

Frequency  of  compressive  strains  in  faulting. DisloCatioUS  of  the  Carth's  SUrfaCe  may 

no  doubt  occur  under  the  most  various  dynamical  conditions,  and  no  gen- 
eral law  can  be  laid  down  as  to  the  presence  or  absence  of  tangential  press- 
ure. It  is  evident,  however,  that  the  lateral  extension  of  a  faulted  ai-ea  is 
increased  by  faulting  whenever  the  hanging  wall  sinks  or  the  foot  wall  rises. 
If  A  is  one-half  of  the  total  slip  measured  on  the  dip  of  the  fissure,  the  in- 
crease of  horizontal  distance  between  any  two  points  on  the  logarithmic 
surfaces  of  the  rising  and  sinking  countries  respectively,  so  far  removed 
from  the  fault  plane  as  to  occupy  positions  which  are  sensibly  on  the  asymp- 
totes of  the  curves,  will  be 

2  A  cos  /?. 

It  is  evident  that  this  increase  in  lateral  extension  will  be  accompanied 
by  lateral  pressure  and  consequent  friction,  unless  the  fault  is  the  result  of 
a  tangential  tensile  strain.  The  general  theories  of  dynamical  geology,  and 
the  study  of  sedimentary  rocks,  however,  show  that  strains  in  the  earth's 
crust  are  commonly  compressive. 

Surface  produced  when  the  fissure  is  a  plane.— It  haS  beCU   sllOWU  that  Uuder  Certain 

conditions  the  sui-face  line  of  the  cross-section  at  any  point  of  a  faulted 
country  will  be  a  logarithmic  curve,  or  a  combination  of  two  logarithmic 
curves.     If  therefore  the  fault  fissure  intersects  the  earth's  plane  surface  on 


STRUCTUEAL  RESULTS  OF  FAULTING. 


177 


a  straight  line,  the  faulted  surface  will  be  that  which  would  be  generated 
by  the  horizontal  movement  of  the  logarithmic  curve  or  curves  along  the 
^-axis  of  the  equation 

y=:A{mr'^ — 1)  —  a;tanS- 

and  in  the  case  of  a  double  curve  in  an  area  of  a  single  rock,  or  of 
rocks  with  the  same  coefficient  of  friction,  this  ^-axis  will  be  found  at  an 
elevation  equal  to  half  the  vertical  distance  between  the  asymptotes  of  the 
curves. 

Surface  produced  when  the  fissure  is  not  a  plane. CommOUly,  howeVCr,.  the  intersec- 
tion of  a  fault  fissure  with  the  earth's  surface  is  not  a  straight  line,  but  an 
undulating  or  broken  one.  If  we  still  suppose  the  original  sui'face  of  the 
area  a  plane,  the  surface  after  faulting  will  be  that  which  would  be  gener- 
ated by  the  movement  of  the  logarithmic  curve  or  curves  along  the  broken 
or  undulating  line  corresponding  to  the  ^-axis,  and  this  line  will  be  the  locus 


Fio.   10. — Cimlonr  iiiiip  of  a  l'aiiltf<l  siiif'aci'. 

of  the  point  of  inflection  of  the  double  curve.  The  line  corresponding  to 
the  tff-axis  will  then  be  the  intersection  of  a  plane  parallel  to  the  original 
surface  of  the  earth  with  the  surface  as  modified  by  the  fault,  and  if  the 
original  surface  was  level,  the  intersection  will  be  a  contour.  Each  inflec- 
12  0  L 


178  GEOLOGY  OF  TQE  COMSTOCK  LODE. 

tion  of  the  trace  of  tlie  fissure  on  the  original  surface  concave  toward  the 
lower  country  will  be  represented  on  the  faulted  surface  by  a  ravine,  and 
each  inflection  convex  toward  the  lower  country  will  result  on  the  faulted 
surface  in  a  ridge. 

Fig.  1 0  shows  a  contour  map  of  the  country  shown  in  Fig.  8,  the  fissure 
having  reached  the  original  flat  surface  of  the  earth  on  the  undulating  line 
AB. 

It  is  evident  that  if  the  form  of  the  trace  were  capable  of  expression 
by  an  algebraic  equation,  the  equation  of  the  faulted  surface  could  be  im-  , 
mediately  deduced,  but  such  cases  are  not  likely  to  occur,  as  deviations  of 
the  trace  from  the  right  line  are  probably  due  to  local  variations  in  the 
physical  properties  of  the  rock.  Even  when  the  original  surface  was  irreg- 
ular the  same  law  holds,  vuifatis  mutandis;  for  the  locus  of  the  point  of 
inflection  of  the  double  logaiithmic  curve  will  still  be  parallel  to  the  trace. 
The  edges  of  the  sheets  on  each  side  of  the  fault  will  be  parallel  to  the 
locus  of  the  point  of  inflection,  and  where  this  is  a  contour  they  will  also 
be  contours. 

It  frequently  happens  that  the  dip-line  of  a  fissure  is  straight  and  nearly 
constant  for  long  distances  from  the  surface,  while  the  strike  varies.  When 
this  is  the  case  the  intersection  with  the  foot  wall  of  a  sui'face  parallel  to 
the  original  surface  at  any  depth  below  it  will  give  tU.e  same  line,  and  if  the 
locus  of  the  point  of  inflection  of  the  sui-face  curve  is  a  contour,  the  contour 
of  the  foot  wall  of  the  fissure  at  any  point  will  be  identical  with  it  and 
with  those  of  the  altered  surface,  as  far  as  the  faulting  action  extends 
unmodified. 

Fissures  into  the  hanging  wall. — TliB  diagrams  sliowat  a  glauce  that  when  a 
fault  takes  place  under  the  conditions  specified,  the  rock  of  the  lower  coun- 
try near  the  fault,  as  seen  in  cross-section,  assumes  the  form  of  a  sharp 
wedge,  which  is  exposed  to  the  same  heavy  pressure  as  the  rock  at  greater 
depths.  In  an  actual  case  in  nature,  it  is  scarcely  possible  to  suppose  that 
this  wedge  would  remain  intact.  A  very  slight  obstruction  to  the  smooth 
rise  of  the  foot  wall  would  produce  a  crack  across  this  edge  at  some  consid- 
erable angle  to  the  dip  of  the  fissure,  and  such  a  crack  might  very  probably 
be  held  permanently  open  b}'  fragments  of  rock.     Fissures  diverging  into 


STEUCTUEAL  EESULTS  OF  FAULTING. 


179 


the  hanging  wall  might  not  unlikely  form  at  greater  depths  as  well,  but 
would  partly  close  again,  leaving  behind  only  openings  of  limited  size, 
because  the  pressure  and  motion  of  the  supei'incumbent  mass  would  suffice 
to  grind  to  powder  most  of  the  intervening  fragments. 

Relation  of  chimneys  to  surface  topography. If  in   faulting,  the  rislng  COUntry  shiftS 

in  the  direction  of  the  strike  of  the  fissure,  of  course  chimneys  will  form 
where  the  strike  undulates.  Where  the  surface  is  modified  by  faulting  in  the 
manner  discussed,  such  chimneys  will  always  lie  on  the  same  side  of  ravines 
on  the  surface,  and  opposite  them  will  be  found  crushed  ground  arising 
from  the  pressure  of  the  walls  upon  one  another. 

Infrequency  of  a  rise  of  the  hanging  wall, TlirOUghoUt    the    foregolug    dlsCUSSioU    I 

have  supposed  that  the  relative  movement  of  the  foot  wall  of  the  fissure  was 
upward,  according  to  the  well-known  empirical  rule.  Were  the  reverse 
case  to  occur,  the  resulting  curve  would  still  be  a  logarithmic  one,  but 
would  be  constructed  in  the  acute  angle  between  the  fault  line  and  the 
asymptote  parallel  to  the  original  surface, 
and  unless  faulting  has  gone  on  but  to  a  very 
slight  extent,  or  unless  the  fault  line  dips  at 
very  close  to  i)0°,  the  resulting  surface  will 
not  merely  be  precipitous,  but  form  a  reen- 
trant curve,  and  the  upper  country  will  over- 
hang the  lower  (Fig.  11).  Countless  faults 
have  been  formed  in  past  geological  eras, 
the  surface  indications  of  which  have  been 
utterly  obliterated,  but  there  must  be  a  very 
great  number  which  still  exliibit  their  features  ^^^^-  n-Kise  of  the  banging  waii. 
in  a  recognizable  form;  and  if  it  were  a  usual  thing  for  the  hanging  wall 
to  rise,  overhanging  surface  would  not  form  one  of  the  rarest  of  topograph- 
ical phenomena. 


Applications  of  the  theory  to  the  Comstock,  and  other  instances. — The  evi- 
dences, already  alluded  to,  of  the  division  of  the  east  and  west  country 
of  the  Comstock  Lode  into  parallel  sheets  lend  probability  to  the  suppo- 
sition that  the    faulted    structure  of  the  central    portion  of  the  vein  may 


180  GEOLOGY  OF  THE  COMSTOCK  LODE. 

come  under  the  conditions  which  have  been  explained  in  the  preceding 
portion  of  tin's  chapter,  and,  as  a  matter  of  fact,  if  the  Sutro  Tunnel  sec- 
tion be  taken  as  a  representative  one,  it  is  easy  to  find  a  logarithmic  curve 
which  shows  a  close  coincidence  with  the  surface.  The  eastern  and  western 
branches  of  the  curve  referred  to  the  fault  line,  and  a  perpendicular  to  the 
fault  line  at  the  cropping  of  the  vein  are,  respectively, 

2/,=:147()"-  (1.00161-"—!)— iCi  tan  44°  27', 

2/2  =  1470"-  (1  -1.00298^0+^2  tan  44°  27'. 

Knowing  these  values,  the  experiment  on  slips  of  paper  can  be  modi- 
fied to  obtain  a  corresponding  result.  The  only  change  needful  is  to  pile 
the  slips  in  such  a  way  that  their  ends  instead  of  falling  in  a  vertical  plane 
will  lie  in  a  plane  forming  an  angle  of  45°  33'  with  the  table.  The  result 
is  a  curve,  which,  when  plotted  on  the  assumption  of  a  suitable  thickness  of 
the  sheets,  is  indistinguishable  from  that  of  one  or  other  of  the  above  equa- 
tions. Precisely  as  in  the  former  experiment,  too,  the  position  of  the  asymp- 
tote precludes  the  supposition  that  the  curve  is  hyperbolic.  There  is, 
therefore,  very  strong  reason  to  believe  that  the  Sutro  section  surface  line 
is  composed  of  two  logarithmic  curves,  and  no  reason  known  to  me  to  sup- 
pose that  it  is  not. 

Atlas-plate  VII.  shows  the  surveyed  surface  line  of  the  Sutro  section 
plotted  from  the  contour  map,  and  in  the  same  figure  the  curve  plotted 
from  the  equations  given  above.  The  same  plate  also  shows  the  curves 
represented  by  the  equations  plotted  by  themselves  with  their  axes  and 
asymptotes,  and  the  curve  obtained  from  experiment.  By  comparing  the 
surveyed  line  with  the  surface  maps,  it  will  appear  that  its  deviations  from 
the  curve  given  by  the  equations  are  the  evident  results  of  plainly  limited 
erosion,  the  section  crossing  two  considerable  ravines  in  the  east  country, 
and  passing  along  the  flank  of  another  in  the  west  country. 

Constants. — Tlic  dislocatlou  measured  on  the  dip  of  the  lode  is  2  A,  or,  for 
the  present  case,  2,940  feet.  The  dip  of  the  lode  at  this  section  is  43°,  and 
the  dislocation  measured  vertically  is  therefore  2,005  feet.  The  angle  S-  is 
44°  27'  and  6  is  therefore  2°  33',  or  the  original  surface  sloped  contrary  to 
the  dip  at  this  angle.     The  natural  unit  of  the  east  curve  is  2,012  feet,  and 


STRCJCTURAL  EESULTS  OF  FAULTING.  181 

if  the  equation  is  referred  to  the  asymptote  and  a  Hne  parallel  to  the  fault 
line  and  crossing  the  asymptote  at  274  feet  west  of  the  fault  line,  it  becomes 
for  tlie  natural  unit, 

The  natural  unit  of  the  west  curve  is  1,085  feet,  and  if  it  be  referred 
to  its  asymptote  and  a  line  parallel  to  the  fault  line  and  crossing  the  asymp- 
tote at  a  point  143  feet  west,  its  equation  is 

y=  — iO^ 

The  equation  of  the  tangent  for  the  Sutro  section  values  shows  that 
the  horizontal  point  of  the  east  curve  is  at  2,840  feet  from  the  fissure  meas- 
ured on  a  line  parallel  to  the  asymptote,  and  that  of  the  west  curve  at  1,820 
feet  measured  in  the  same  way.  The  tangent  to  the  east  curve  at  the  fault 
makes  an  angle  of  26°  with  the  horizontal,  while  the  tangent  to  the  west  curve 
makes  an  angle  of  32°.  This  sudden  increase  of  inclination  immediately 
west  of  the  croppings  is  a  familiar  feature  of  the  landscape  in  Virginia. 
Had  the  diorite  been  separated  into  plates  of  the  same  thickness  as  those 
of  the  east  country,  the  two  curves  would  have  had  a  common  tangent  at 
the  croppings. 

The  position  of  the  points  of  greatest  curvature  presents  no  significant 
peculiarity,  so  far  as  I  am  aware,  and  is  expressed  by  a  somewhat  involved 
logarithmic  function.  This  point  in  the  east  curve  is  at  a  distance  of  686 
feet  from  the  fault  plane,  measured  on  the  asymptote.  In  the  west  curve 
it  lies  at  951  feet  from  the  same  plane.  The  values  of  the  minimum  radii 
are  6,640  feet  and  3,580  feet  in  the  east  and  west  curves,  respectively. 
These  radii  are  simply  and  directly  proportional  to  the  natural  units  of  the 
curves. 

Topography  chiefly  due  to  faulting. — The  wcst  cropplngs  of  the  CoMSTOCK,  from  the 
Bullion  to  the  OpJiir,  are  nearly  horizontal,  and  the  original  surface,  as  has 
been  shown,  sloped  to  the  west  at  an  angle  of  only  two  and  a  half  degrees. 
The  theory  of  faulting  propounded  would  therefore  lead  one  to  expect  a 
pretty  close  agreement  between  the  contours  of  the  faulted  slope  and  those 
of  the  west  wall;  for  on  the  Sutro  Tunnel  section,  at  least,  there  is  evidence 
of  but  slight  erosion.     Such  an  agreement  appears  from  a  comparison  of 


182  GEOLOGY  OF  THE  COMSTOCK  LODE. 

the  horizontal  sections  with  the  surface  map,  and  has  long  been  well  rec- 
ognized among  those  who  have  had  to  do  with  the  mines. 

The  ravines  which  furrow  the  range  are  not  therefore  the  result  of 
erosion,  but  of  faulting.  Once  formed  through  the  dislocation  of  the 
country,  they  have,  of  course,  received  the  drainage,  and  have  been  modi- 
fied thereby  to  some  extent. 

East  vein. — It  lias  beeu  shown  that  even  if  a  fault  takes  place  on  a  fissiare 
perpendicular  to  an  original  surface,  the  hanging  wall  will  assume  the  shape 
of  a  sharp  wedge,  and  that  under  the  conditions  of  pressure  necessary  to 
produce  a  logarithmic  surface,  it  is  unlikely  that  this  wedge  would  remain 
intact.  Such  a  fractui-e  occurred  in  the  faulting  of  the  Comstock,  and 
opened  the  famous  "east  vein",  from  which  a  large  part  of  the  ore  produced 
has  been  extracted.  Baron  von  Richthofen  regarded  this  structure  as  a 
result  of  faulting,  and  as  a  surface  phenomenon.  I  have  simply  shown  in 
addition  how  the  east  country  came  to  assume  the  tapering  form  most 
favorable  to  such  a  fracture. 

Origin  of  the  sheeted  structure.     Theory  of  eruptive   stratification. i  lie      CharaCtCr      01     thC 

sheets  of  rock  into  which  the  walls  of  the  Comstock  are  divided  is  an  open 
question,  for  one  observer  has  maintained  that  they  form  a  series  of  thin, 
bedded,  regular  layers  of  rock,  presenting  a  fine  example  of  eruptive  strati- 
fication. It  is  true  that  in  confined  spaces  in  several  of  the  rocks  a 
stratified  or  laminated  texture  is  visible;  but  in  the  half-dozen  such  cases 
known  to  me  the  phenomenon  extends  for  very  short  distances,  often  only  a 
few  feet,  and  appears  to  be  the  result  of  some  local  variation  in  the  compo- 
sition of  the  rock;  for  not  only  can  I  perceive  no  general  uniformity  in  the 
direction  of  the  layers  in  these  different  spots,  but  I  have  a  single  hand- 
specimen  which  shows  portions  of  two  sets  of  them  at  an  angle  of  nearly 
90°  to  one  another.  These  occurrences,  however,  cannot  be  meant  in  the 
statement  referred  to,  for  they  are  rare.  As  applied  to  the  great  mass  of 
rock  I  am  also  unable  to  agree  with  it.  To  me  it  is  nearly  inconceivable 
that  a  granular  crystalline  rock  like  the  diorite  of  Mount  Davidson,  con- 
taining only  crystals  of  "secondary  consolidation,"  should  ever  have  been 
sufficiently  fluid  to  permit  of  eruptive  bedding  The  face  of  Mount  David- 
son shows  no  lamination,  though  the  division  into  parallel  sheets  is  strikingly 


STRUCTURAL  RESULTS  OF  FAULTING.  183 

apparent.  The  surfaces  of  the  sheets  in  the  same  locah'ty  are  not  similar 
to  those  commonly  formed  by  bedding,  and  are  indisting-nishable  from  frac- 
tures, nor  is  the  persistence  of  the  sheets  comparable  with  that  of  sedi- 
mentary strata.  The  McKibhen  Tunnel  in  Spanish  Ravine  passes  through 
diorites  in  part  somewhat  porphyritic,  in  part  of  the  dark,  highly  horn- 
blendic  variety.  A  quartz  seam  is  cut  by  the  tunnel,  but  no  dikes  of  later 
rocks.  There  is  a  greater  superficial  resemblance  to  a  bedded  structure 
here  than  on  Mount  Davidson,  but  close  examination  shows  that  most  of  the 
apparent  differences  in  color  and  texture  are  referable  to  degrees  of  decom- 
position. Decomposition  has  set  in  from  the  partings  of  the  sheets  of  rock, 
often  leaving  the  central  portion  of  a  sheet  less  affected  than  its  faces.  In 
the  diabase,  hornblende-andesite,  and  augite-andesite  of  the  east  country, 
the  phenomena  are  similar.  There  is  ample  evidence  of  fracture  and  of 
decomposition  following  lines  of  fracture.  Sometimes  individual  sheets  or 
portions  of  sheets  have  in  a  measure  escaped  decomposition  on  account  of 
the  presence  of  protecting  clay  seams  and  the  like,  and  these  have  been 
mistaken  for  dikes,  or  flows  of  andesite  or  other  rock;  but  careful  examina- 
tion  shows  that  they  differ  only  in  the  degree  to  which  they  have  yielded  to 
decomposing  agencies,  and  in  no  other  respect.  The  partings  are  not  such 
as  we  should  expect  in  bedded  flows.'  There  is  no  trace  of  lamination 
except  the  irrelevant  local  occurrences  mentioned,  and  while  it  might  well 
be  that  the  greater  part  of  the  seams  had  been  reopened  by  upheaval,  it 
cannot  be  supposed  that  no  adherent  laminae  would  escape  separation.  In 
short,  my  observations  wholly  fail  to  accord  with  the  hypothesis  that  these 
rocks  were  laid  down  in  horizontal  beds,  and  afterward  tilted.  Even  if 
observation  furnished  considerable  grounds  for  such  an  interpretation  of  the 
facts,  I  should  hesitate  to  accept  an  explanation  which  appears  to  me  wholly 
at  variance  with  what  we  know  of  the  occurrence  of  similar  rocks  else- 
where.' The  deposition  of  a  single  igneous  rock  over  several  square  miles, 
in  thin  horizontal  beds,  implies  a  watery  fluidity  and  a  very  high  specific 
heat.     So  far  as  I  know  only  one  or  two  of  the  later  volcanic  rocks  are 

'Mr.  Church,  indeed,  states  {I.  c,  p.  153)  th.at  "diorite  is  one  of  the  fine-graiued,  thin,  ruuning 
hivas."  But  he  cites  uo  authorities  for,  or  instances  in  proof  of,  this  statement,  which  is  .at  variance 
■with  the  coranionly  accepted  opinion,  and  with  the  indications  of  its  composition  and  micro-structure. 


184  GEOLOGY  OF  THE  GOMSTOCK  LODE. 

known  to  flow  in  such  a  manner,  and  these  only  under  exceptional  condi- 
tions, for  even  basalt  commonly  accumulates  in  large  masses  around  the 
orifices  from  which  it  issues;  nor  am  I  aware  of  any  distinct  evidence  that 
the  granitoid  rocks  have  ever  flowed  like  a  lava,  or  reached  a  higher  degree 
of  fluidity  than  the  plastic  state. 

Energy  displayed  in  the  fault  on  the  Comstock. We    haVe    UO    meanS    of   reducing    tO 

known  units  the  pressure  and  resultant  friction  which  accompanied  the 
faulting  action  on  the  Comstock,  but  the  imagination  at  least  may  be 
brought  to  bear  upon  the  subject  by  considering  the  amount  of  disloca- 
tion. If  the  west  country  is  supposed  to  have  revolved  about  a  distant 
fixed  fulcrum,  through  a  sufficient  angle  to  account  for  its  present  relative 
elevation,  then  the  east  country  must  have  been  pushed  bodily  eastward  for 
a  distance  of  2,150  feet.  The  maps  and  sections  show  that  certainly  not 
less  than  a  cubic  mile  of  rock  must  have  been  thus  driven  out  of  place  in 
spite  of  all  opposition,  and  the  amount  of  horizontal  dislocation  involved  is 
not  lessened  by  supposing  the  west  country  to  have  moved  instead  of  the 
east.  Compared  with  the  energy  necessary  to  produce  such  a  movement, 
that  requisite  merely  to  raise  each  of  the  sheets  composing  the  mass,  in 
opposition  to  friction  through  a  mean  distance  of  about  150  feet,  certainly 
seems  small. 

Dynamical  theory  of  sheets. — I  havc  showu  that  the  teudcucy  of  the  faulting 
movement  is  to  separate  sheets  of  rock,  and  that  sheets  thus  separated  will 
arrange  themselves  along  the  logarithmic  curve  when  divided  from  the 
mass.  The  possibility  thus  presented  does  not  conflict  with  my  observa- 
tions, and  I  am  led  to  the  belief  that  the  sheeted  structure  of  the  east  and 
west  country  is  due  to  the  formation  of  fractures  parallel  to  the  faulting 
surface,  and  that  these  fractures  are  the  result  of  faulting  under  intense 
lateral  pressure. 

Inferences  from  the  fault  as  to  the  age  of  the  Lode. SomC  light  is  thrOWU  UpOU  tho  age 

of  the  Comstock  as  an  oi'e  vein  by  the  relations  of  the  fault  to  the  ore,  and 
to  the  erosion.  The  "  east  vein,"  being  a  secondary  fissure,  cannot  have 
formed  till  faulting  had  made  considerable  progress,  while  the  crushed  con- 
dition of  the  quartz^  and  the  phenomena  attending  it  show  that  faulting 

'  The  evidence  that  the  "sugary  quartz"  is  really  crushed  will  he  giveu  later. 


STRUCTURAL  RESULTS  OF  FAULTING.  185 

action  has  succeeded,  as  well  as  preceded,  the  deposition  of  the  ore  and 
gangue.  The  regularity  of  the  curve,  on  the  other  hand,  shows  that  the  origi- 
nal surface  line  along  the  Sutro  section  was  sensibly  straight^  and  lay  on  a 
gentle  western  slope.  The  agreement  of  the  contours  of  the  range  with 
those  of  the  west  wall  and  of  the  cross-sections  with  the  curve  obtained 
from  theoretical  considerations,  proves  that  the  erosion  since  the  commence- 
ment of  the  faulting  action  is  sensible  (on  a  scale  of  800  feet  to  the  inch) 
only  where  most  intensified — i.  e.,  in  the  ravines.  The  faulting  and  the  depo- 
sition of  ore  have  therefore  occurred  since  the  Distkict  was  subjected  to  any 
considerable  amount  of  general  degradation.  The  level  condition  of  the 
country  prior  to  the  fault  appears  to  me  probably  the  result  of  erosion,  and 
if  so  the  District  must  have  been  a  plateau  or  a  high  mountain  valley — in 
short,  an  area  of  denudation. 

Fault  probably  the  result  of  a  rise  in  the  west  country. It  Is  pd'hapS  impOSslblc  tO  demon- 
strate whether  the  absolute  movement  involved  in  the  faulting  was  the  rise 
of  Mount  Davidson,  or  a  sinking  of  the  east  country.  If  the  east  country 
has  sunk,  the  former  level  near  the  middle  of  the  Lode  must  have  been 
nearly  that  of  Mount  Davidson,  and  the  District  must  have  occupied  the 
crest  of  a  rather  sharp  undulation  running  nearly  east  and  west.  If  the 
main  movement  was  an  uplift  of  Mount  Davidson,  and  its  neighbors  to  the 
north  and  south,  the  original  general  level  was  about  that  of  the  present 
country  east  of  the  Lode.  The  District  must  then  have  been  near  the  top 
of  a  gentle  undulation  approximately  parallel  to  the  Sierra.  The  latter 
supposition  accords  with  the  general  character  of  the  present  topography  of 
the  Great  Basin  area  much  better  than  the  former,  and  seems  to  me  much 
more  probable  on  general  as  well  as  local  grounds. 

Diminution  of  evidence  of  fault  near  the  ends  of  the  Lode. To     tho     UOrtll     aud     SOUth     of 

Mount  Davidson  the  evidence  of  faulting  diminishes.  From  the  Overman 
far  into  the  Sierra  Nevada  claim,  a  distance  of  two  and  one-third  miles,  the 
amount  of  fault  has  been  great,  and  the  indications  are  unmistakable.  Be- 
yond these  points  the  disturbance  of  equilibrium  has  been  to  some  extent 
adjusted  in  a  different  manner.  This  is  partly  indicated  on  the  surface  map 
by  the  union  of  the  andesite  fields,  which  are  separated  opposite  the  middle 
portion  of  the  Lode  by  diorites.     Towards  the  ends  of  the  Lode  the  dynamic 


•  I 


186  GEOLOGY  OF  THE  COMSTOCK  LODE. 

action  seems  to  have  been  distributed  in  part  by  a  forking  of  the  fissure, 
and  in  part  by  the  formation  of  east-and-west  cracks. 

Coefficientof  friction  of  rocks  involved  in  the  fault. Tho  rOcks    iuVOlved  in   tho  faulting 

action  on  the  Sutro  section  are  diorite,  diabase,  hornblende-andesite,  and 
augite-andesite.  They  must  all  have  sensibly  equal  coefficients  of  friction, 
for  the  curve  in  the  western  diorite  is  apparently  continuous  with  that  of 
the  other  rocks  which  lie  east  of  the  vein,  and  there  is  no  evidence  of  dis- 
continuity in  the  eastern  curve  as  it  passes  the  contacts.  All  the  east  rocks, 
too,  appear  to  divide  into  plates  of  the  same  thickness,  while  the  diorite  has 
split  into  sheets  of  less  than  half  that  of  the  others. 

Rules  applicable  to  prospecting  in  uneroded  districts. It  is,  of  COUrSe,  mOSt  Uulikcly  that 

the  CoMSTOCK  is  the  only  vein  in  which  the  deposition  of  ore  is  recent,  and 
has  been  accompanied  by  faulting,  and  some  conclusions  as  to  the  occur- 
rence of  veins  in  such  cases  may  be  welcome  to  some  of  the  readers  of  this 
paper. 

In  a  locality  modified  by  faulting  action  under  lateral  pressure,  the  fact 
will  appear  in  the  parallelism  of  the  exposed  edges  and  faces  of  rock-sheets. 

If  erosion  has  not  seriously  modified  the  surface  resulting  from  the 
faulting  action,  the  logarithmic  curve  will  be  recognizable  to  the  observer 
looking  in  the  direction  of  the  strike. 

The  main  cropping  of  the  vein  is  to  be  sought  at  the  point  of  inflection 
of  the  curve,  which  will  be  found  nearly  or  exactly  midway  between  the 
top  and  bottom  of  the  hillside.  One  or  more  secondary  vein  croppings 
should  be  looked  for  below  the  main  cropping,  and  these,  so  far  as  yield  is 
concerned  (but  not  in  regard  to  location  of  claim),  may  prove  more  impor- 
tant than  the  main  cropping. 

The  dip  of  the  vein  will  be  to  the  same  quarter  as  the  slope  of  the  sur- 
face, but,  of  course,  greater  in  amount.  The  flatter  the  surface  curve,  the 
smaller  the  angle  of  dip  will  be.  The  mean  strike  will  be  nearly  or  quite 
at  right  angles  to  the  direction  of  the  spurs  and  ravines  of  the  faulted  area. 

If  besides  the  movement  of  one  or  other  wall  in  the  azimuth  of  the 
dip,  there  has  been  a  dislocation  in  the  direction  of  the  strike,  chimneys  will 
open,  all  of  them  on  the  same  side  of  the  diff'erent  ravines.     Surface  evi- 


STRUCTURAL  RESULTS  OF  FAULTING.  187 

deuces  will  often  enable  the  prospector  to  determine  on  which  side  the 
chimneys  are  to  be  found.  On  the  barren  sides  evidences  of  crushing  and 
of  closure  of  the  fissure  are  probable. 

The  fissure  is  more  likely  to  have  a  constant  dip  (barring  the  second- 
ary off"shoots)  than  a  constant  strike ;  but,  of  course,  irregularities  in  dip 
like  those  in  strike  will  open  chambers  which  may  be  productire. 

Ofishoots  into  the  hanging  wall  may  occur  at  any  depth,  but  none  except 
those  near  enough  to  the  main  cropping  to  reach  the  surface,  where  it 
has  a  very  considerable  slope,  are  likely  to  be  continuous. 

Application  of  theory  to  landslips. — Bcsidcs  the  dccp-scated  fissures  produced  by 
profound  disturbances  of  the  earth's  crust,  there  are  cortiparatively  super- 
ficial phenomena  which  seem  to  come  under  the  laws  deduced  in  this  chap- 
ter. In  regions  where  the  soil  is  deep  and  covered  with  low-growing 
vegetation,  such  as  grass,  the  details  of  the  topography  are  not  molded  by 
the  direct  action  of  the  rain,  but  by  landslips ;  oftentimes,  indeed,  of  very 
small  extent,  but  repeated  or  increased  year  after  year.  The  hanging  wall 
of  such  landslips  commonly  separates  into  distinct  layers,  as  has  been  stated 
in  a  preceding  paragraph.  These  sheets  must  arrange  themselves  on  the 
locus 

if  the  arguments  presented  on  p.  I(i4,  et  seq.,  are  correct.  A  yearly  repeti- 
tion of  this  action,  sometimes  modifying  the  hanging  wall  and  sometimes 
the  foot  wall  of  the  slips,  will  eventually  give  the  whole  topography  a  log- 
arithmic character;  even  the  position  of  the  gullies,  and  consequently  the 
lines  of  direct  erosion,  being  determined  as  indicated  on  page  177.  The  simi- 
larity between  some  of  the  logarithmic  curves  illustrated  in  this  chapter  and 
the  slopes  of  the  gently-rounded  hills  common  in  grassy  regions  with 
deep  soil,  needs  only  to  be  suggested. 


OHAPTEE    V.     . 

THE  OCCURRENCE  AND  SUCCESSION  OF  ROCKS. 

Methods  of  determining  succession. — Determinations  of  the  order  of  succession  of 
eruptive  rocks  involve  considerable  difficulties.  Superimposition  alone  is 
an  insufficient  indication  of  relative  age,  for  intrusions  and  laccolitic  accu- 
mulations of  younger  rocks  may  underlie  older  ones.  Neither  are  inclu- 
sions of  one  rock  in  another  always  a  safe  guide.  Cases  are  not  unknown 
where  intrusive  masses  of  a  younger  rock  in  an  older  might  readily  be  mis- 
taken for  inclusions  of  an  older  rock  in  a  younger  one.  I  have  even  ob- 
served instances,  though  not  in  the  Washoe  District,  of  slabs  of  older 
rocks  embedded  in  later  eruptions  in  such  a  manner  that  but  for  other  and 
overwhelming  evidence  as  to  the  order  of  succession,  they  might  have  been 
interpreted  as  dikes  of  the  older  rock  in  the  younger.  Moreover,  when  the 
rocks  in  question  are  closely  allied,  as  is  very  frequently  the  case,  local 
modifications  of  one  rock  may  readily  be  confounded  with  inclusions  of  a 
different  but  similar  species.  Such  an  error  is  peculiarly  likely  to  occur 
where  there  is  brecciation.  As  has  been  pointed  out  on  page  82,  masses  of  a 
single  rock  subjected  to  partial  decomposition  may  also  simulate  inclusions 
or  dikes  of  one  rock  in  another.  Thus  while  at  first  sight  it  might  appear 
that  dikes  and  inclusions  furnish  the  most  unimpeachable  evidence  of  suc- 
cession, this  class  of  evidence  is  peculiarly  deceptive  except  where  the  rocks 
are  fresh  and  characteristic,  the  exposure  perfect,  and  the  cases  abundant. 
Where  any  of  the  rocks  are  very  recent,  evidences  of  erosion  form  an  im- 
portant argument  as  to  succession,  as  will  be  seen  from  the  remarks  on  the 
later  hornblende-andesite. 

No  single  method  of  determining  the  succession  of  eruptive  rocks  is 
ordinarily  sufficient,  and  due  weight  must  be  given  to  all  the  facts  bearing 

188 


OCOUEEENCE  AND  SUCCESSIOlSr  OF  EOGKS.  189 

upon  their  relative  age.  Difficulties  in  the  determination  of  succession, 
however,  are  not  peculiar  to  the  geology  of  massive  rocks;  for  there  are 
many  instances  of  the  reversal  of  sedimentary  strata,  and  with  sufficient 
care  the  order  of  succession  of  eruptives  can  generally  be  established  with 
as  much  certainty  as  can  that  of  sedimentary  rocks  in  greatly  disturbed 
areas. 

Order  of  succession. — Thc  ordcr  iu  which  the  rocks  of  the  Washok  District 
have  appeared  upon  the  surface  is  as  nearly  as  can  be  ascertained  the  fol- 
lowing: 

Granite, 

Metamorphics, 

Granular  diorites, 

Porphyritic  diorites, 

Metamorphic  diorites, 

Quartz-porphyry, 

Earlier  diabase. 

Later  diabase  ("black  dike"), 

Earlier  hornblende-andesite, 

Augite-andesite, 

Later  hornblende-andesite. 

Basalt. 

It  is  possible  that  strata  since  metamorphosed  may  have  been  laid  down 
upon  the  diorite  as  well  as  previous  to  it.  The  evidence  of  the  succession 
of  diabase  to  quartz-porphyry  would  be  more  satisfactory  if  the  contact 
between  them  were  more  extensive,  and  of  the  age  of  the  basalt  there  is  no 
direct  evidence  except  that  it  is  later  than  earlier  hornblende-andesite. 
The  other  points  as  to  succession  are  clearly  established.  One  of  the  most 
interesting  is  the  occurrence  of  hornblende-andesite  after  as  well  as  before 
augite-andesite,  proving  a  recurrence  in  the  character  of  eruptions.  It  thus 
has  a  direct  bearing  upon  the  general  theory  of  the  succession  of  volcanic 
rocks.  In  the  following  pages  some  notes  are  presented  on  the  occurrence 
and  distribution  of  each  of  the  series. 


190  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Granite. — Granite  is  extensively  develojied  to  the  west  of  the  Virginia 
Range,  but  reaches  the  surface  in  the  Washoe  District  only  in  a  single 
small  area  near  the  Red  Jacket  mine,  C.  D.  6.  It  occupies  a  considerable 
space  beneath  tlie  surface,  however,  for  it  has  been  met  in  the  Baltimore  and 
the  Bock  Island,  and  by  a  tunnel,  just  beyond  the  limits  of  the  map,  to  the 
northwest  of  the  Florida. 

The  granite  must  fall  away  very  rapidly  to  the  north  and  east,  or  it 
would  be  encountered  in  the  Gold  Hill  mines.  Whether  this  is  the  conse- 
quence of  a  fault  or  of  a  steep  slope,  there  is  no  opportunity  for  deciding. 
Near  the  Bed  Jacket  the  granite  here  and  there  shows  partings  which  might 
be  remains  of  a  former  stratification ;  but  a  similar  system  of  parallel  cleav- 
ages is  not  uncommon  over  small  areas  in  rocks  of  an  unquestionably 
eruptive  character,  and  I  met  with  nothing  which  could  be  cited  as  definite 
proof  of  a  sedimentary  origin. 

In  the  Wales  Consolidated,  granite  is  directly  overlain  by  metamorphic 
diorite,  at  the  Bock  Island  by  schists  and  limestones,  and  at  the  Baltimore 
apparently  by  eruptive  diorite,  metamorphics,  quartz-porphyry,  and  augite- 
andesite.  It  must,  therefore,  have  been  denuded  to  a  considerable  ex- 
tent before  each  of  several  eruptions.  It  is  nevertheless  far  fresher  than 
most  of  the  rocks  in  the  District,  and  no  considerable  quantity  of  ore 
has  been  found  associated  with  it,  though  some  metalliferous  quartz  has 
been  met  with  at  its  contact  with  younger  rocks ;  but  traces  of  ore  are  very 
likely  to  occur  at  any  contact  in  a  district  like  Washoe,  where  every 
point  has  been  racked  by  dynamical  action  and  the  whole  subterranean  area 
has  been  flooded  with  mineral  solutions.  It  is  possible  that  ore  similar  to 
the  Justice  body  may  be  found  on  the  contact  between  metamorphic  diorite 
and  granite  south  of  that  mine,  but  there  is  nothing  to  indicate  that  the 
granite  is  likely  to  act  otherwise  than  mechanically  in  the  deposition  of  ore. 

Metamorphics. — Tlicre  Is  a  Small  area  of  distinctly  stratified  rocks  to  the 
south  of  American  Flat,  near  the  Florida.  They  are  limestones  and  mica- 
ceous schists,  badly  broken  and  contorted,  and  much  metamorphosed.  I 
did  not  succeed  in  detecting  anything  like  a  fossil  in  them,  in  spite  of  an  earn- 
est search.     They  are  colored  as  Mesozoicfrom  the  general. analogy  of  this 


OCCURRENCE  AND  SUCCESSION  OF  liOCKS.  191 

portion  of  the  Great  Basin,  aw  elucidated  by  the  Exploration  of  the  Fortieth 
Parallel.  In  a  cut  on  the  American  Flat  road,  just  south  of  tlie  Florida, 
there  occur  two  seams  of  coal-like  matter  half  an  inch  in  thickness.  The 
metamorphics  extend  into  American  Flat  under  the  area  laid  down  -as 
Quaternary,  where  the  detritus  is  too  thick  to  permit  of  tracing  the  con- 
tact between  the  metamorphic  and  eruptive  rocks  with  certainty.  The 
Rock  Island  shaft  is  inaccessible,  but  a  careful  examination  of  the  dump  and 
the  descriptions  of  an  employ^  leave  no  doubt  that  it  passed  through  meta- 
morphics into  underlying  granite.  There  is  nothing  to  show  that  any 
eruptive  rock  other  than  granite  has  been  met  with  at  the  Bock  Island.  A 
little  coal  is  said  to  have  been  found  well  down  towards  the  granite,  and 
was  no  doubt  such  an  occurrence  as  that  mentioned  above.  Metamorphics 
of  the  same  character  appear  to  an  insignificant  extent  north  of  American 
Flat,  and  in  the  Caledonia,  as  is  shown  on  the  section  through  that  mine. 
In  the  Gold  Hill  mines  black  slates  form  the  foot  wall  of  the  Lode  to  a 
large  extent.  Thin  sections  made  across  the  lamination  show  that  the  dark 
color  is  due  to  absolutely  opaque  particles  without  metallic  luster,  and 
these  disappear  on  prolonged  heating  in  an  oxidizing  flame,  but  are  not 
affected  by  acids.  They  are  therefore  graphite.  The  rock  contains  pyrite, 
which  is  very  irregularly  distributed.  The  slate  is  often  confounded  with 
"black  dike"  (younger  diabase),  with  which,  however,  it  shares  only  the 
black  color.  In  a  fairly  good  light  the  slaty  structure  serves  to  distinguish 
it  without  difficulty.  The  diorite  at  the  Yellotv  Jacket  appears  to  overlie 
these  slates,  though  no  single  mine-opening  shows  a  contact.  The  masses 
of  mica-diorite  shown  in  the  Yellow  Jacket  section  can  hardly  be  in  their 
original  position,  though  very  likely  they  have  been  transported  but  a  very 
short  distance;  but  at  the  surface  the  dioritic  mass  is  in  sight  to  within  a  few 
hundred  feet  of  the  Yellow  Jacket,  where  it  seems  to  disappear  under  the 
andesites,  and  it  is  almost  impossible  to  suppose  that  the  great  exposure  of 
slates  in  the  Yellotv  Jacket  and  the  Belcher  is  not  one  surface  of  a  body  which 
extends  beneath  the  neighboring  diorite.  On  the  other  hand,  in  the  Cale- 
donia diorite  underlies  the  metamorphics,  and  it  therefore  seems  probable 
that  the  plastic  diorite  was  forced  horizontally  between  sedimentary  masses 
as  well  as  vertically  to  the  surface  or,  at  all  events,  to  higher  points  than 


192  GEOLOGY  OF  THE  COMSTOCK  LODE. 

any  now  occupied  by  stratified  rocks.  Further  indications  of  sucli  a  history 
are  observable  in  the  Sierra  Nevada,  where  a  thin  and  not  very  extensive 
body  of  highly  crystalline  stratified  limestone  is  completely  inclosed  in 
diorites,  which  are  granular  on  one  side  and  porphyritic  on  the  other.  I 
am  able  to  offer  no  better  suggestion  than  that  this  mass  was  carried  into 
its  present  position  by  the  granular  diorite,  and  covered  over  sooner  or  later 
by  a  porphyritic  outflow. 

Eruptive  diorite. — Besidcs  tlic  dioHtic  mass  forming  Mount  Davidson  and 
the  adjoining  hills,  there  is  somewhat  obscure  surface  evidence  of  a  large 
area  of  this  rock  beneath  later  eruptive  masses.  Near  the  Forman  shaft  are 
several  small  patches  of  mica-diorite,  which,  however,  might  easily  be  passed 
unnoticed;  and  in  the  Flowery  district,  about  a  mile  and  a  half  east  of 
Flowery  Peak,  dioritic  porphyries  again  appear.  Diorites  occur  in  almost 
all  the  CoMSTOCK  mines  from  the  Silver  Hill  north  to  the  Utah,  and  are 
also  found  in  those  of  the  Floweiy  region.  The  dump  of  the  Lady  Bryan, 
for  example,  consists  largely  of  fresh,  coarsely  granular,  quartzose  diorite 
To  the  west  and  northwest  of  Mount  Davidson  it  also  appears  to  be  covered 
by  but  a  thin  cap  of  andesite,  so  that  at  least  two  islands  of  the  older  rock 
are  wholly  surrounded  by  the  5'ounger.  Diorite  forms  the  foot  wall  of  the 
Lode  throughout  the  Vii-ginia  mines  and  is  replaced  in  this  position  by  met- 
amorphics  in  Gold  Hill.  On  the  hanging  wall  it  is  found  in  the  Yellow 
Jacket  in  masses  apparently  displaced,  and  in  the  Sierra  Nevada  and  Utah  it 
forms  both  walls  of  the  fissure  which  has  been  mainly  explored.  Frag- 
mentary masses  also  appear  embedded  in  diabase  at  intermediate  points 
but  not  to  an  important  extent.  Before  the  eruption  of  the  earlier  diabase, 
the  diorite  no  doubt  formed  a  continuous  mass,  partly  overlying  and  partly 
underlying  the  metamorphic  strata,  and  probably  extended  over  the  coun- 
try now  occupied  by  later  rocks  along  the  line  of  the  Sutro  Tunnel.  If  so, 
this  area  has  sunk  under  the  subsequent  outflows,  but  how  far  it  is  as  yet 
impossible  to  say,  though  it  is  a  matter  of  importance  to  the  future  of  the 
Lode.  At  the  time  of  the  faulting  the  whole  west  wall  in  Virginia  and 
Gold  Hill  seems  to  have  risen,  the  dislocating  tendency  having  been  adjusted 
towards  the  ends  of  the  fissure  by  diverging  cracks.  This  action  has  moulded 
the  eastern  face  of  the  range  opposite  Virginia  City  and  the  northern  por- 


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OCOUEEENCE  AND  SUCCESSION  OF  EOCKS.  193 

tion  of  Gold  Hill.  To  the  north  of  the  Union  shaft  the  porphyritic  diorites 
swing-  to  the  northeast.  On  the  surface  they  disappear  under  the  andesites, 
while  underground  the  explorations  north  of  the  OpUr  have  been  almost 
wholly  confined  to  the  dioritic  area,  and  afford  no  means  of  tracino-  the 
extension  of  the  diorites  beneath  the  cap.  Near  where  the  contact  between 
the  diorites  and  the  diabase  probably  occurs  are  the  heavy  croppings  known 
as  the  Scorpion.  Whether  these  actually  correspond  to  the  contact  or  not 
can  only  be  told  by  exploration;  but,  if  not,  that  contact  has  left  no  trace 
upon  the  surface  in  this  region,  which  would  be  very  remarkable  if  the 
deductions  made  in  the  last  chapter  as  to  the  age  of  the  Lode  are  correct. 
It  is  not  unlikely  that  the  dioritic  rocks  are  continuous,  or  nearly  so,  under 
the  Flowery  Eidge,  and  are  thus  connected  with  the  occurrences  at  and 
near  the  Lady  Bryan.  Diorite  seems  to  have  preceded  the  quartz-porphyry, 
for  it  occurs  in  the  Justice,  and  in  the  Caledonia,  beneath  the  porphyry. 

Relations  of  porphyritic  to  granitoid  forms. Thc  TelationS   of   the  dioHtic    pOrphyHeS 

to  the  granular  mass  are  interesting.  The  former  are  constantly  found  over- 
lying the  granular  rock,  but  a  line  of  demarkation  can  seldom  be  drawn, 
transitions  and  mixed  masses  being  of  constant  occurrence.  Roughly  the 
area  between  Bullion  and  Spanish  ravines  is  granitoid,  and  the  masses 
beyond  these  limits  porphyritic;  but  this  is  a  very  rude  approximation,  for 
fine  porphyries  occur  in  the  very  midst  of  the  mass  of  Mount  Davidson, 
and  granular  patches  are  to  be  found  throughout  the  hornblendic  porphyries. 
The  micaceous  porphyries  also  appear  to  overlie  the  hornblendic  variety, 
into  which,  however,  they  merge.  The  conditions  suggest  a  physical  explana- 
tion Some  geologists  now  believe  that  the  crystalline  structure  of  rocks 
depends  solely  on  the  pressure  under  which  they  have  consolidated.  Such  an 
explanation  of  the  present  case,  however,  seems  to  me  unsatisfactory.  The 
variation  in  a  horizontal  direction  is  nearly  as  marked  as  that  in  a  vertical 
line,  and  though  there  is  an  exposure  of  at  least  2,500  feet,  vertically,  allow- 
ing for  the  displacement  by  faulting,  the  deepest  granular  diorites  are  not 
more  coarsely  crystalline  than  those  on  the  top  of  Mount  Davidson.  Nor 
are  the  other  rocks  from  the  bottom  of  the  mines  in  any  perceptible  manner 
different  from  those  collected  at  or  near  the  surface.  The  cause  of  the  differ- 
ence between  the  granular  and  the  porphyritic  diorite,  if  these  rocks  are  ad- 

13   0  L, 


194  GEOLOGY  OF  THE  COMSTOCK  LODE. 

mitted  to  be  of  eruptive  origin,  must,  I  think,  be  sought  in  a  period  anterior  to 
the  extrusion  of  the  mass.  The  granular  diorite  is  composed  of  crystals  of 
"secondary  consolidation,"  interlocking  grains,  the  relative  position  of  which 
cannot  have  changed  subsequent  to  their  formation.  This  rock  must,  there- 
fore, have  crystallized  in  its  present  position,  barring,  of  course,  any  move- 
ments to  which  it  may  have  been  subjected  after  solidification.  The  porphy- 
ries, on  the  other  hand,  are  composed  of  well-developed  crystals  in  a  granu- 
lar groundmass.  These  crystals  must  have  grown  slowly  in  a  magma  suffi- 
ciently fluid  to  permit  of  free  movement,  and  this  condition  is  not  likely  to 
have  been  present  after  eruption.  A  state  of  considerable  fluidity  is  also 
indicated  by  traces  of  bi'ecciation  in  some  of  these  rocks,  and  of  fluidal 
structure  in  the  arrangement  of  microlites  in  a  few  slides.  But  the  strong- 
est evidence  of  a  fluid  condition  is  furnished  by  the  little  dike  close  to  the 
Eldorado  croppings.  The  walls  are  granitoid,  and  the  center  o/  the  dike  is 
semi-porphyritic,  showing  green  fibrous  hornblende  and  a  granular  structure, 
though  some  porphyritical  ciystals  are  imbedded  in  it.  But  for  an  inch 
from  the  walls  of  the  dike  the  rock  is  a  dark,  solid  porphyry  which  contains 
brown  hornblendes,  and  is  in  all  respects  similar  to  the  most  porphyritic 
varieties  found  in  the  District.  The  contact  with  the  walls  is  perfect,  and 
the  occurrence  admits  of  no  natural  explanation  but  that  of  a  hot  intrusive 
fluid. 

Hypothesis  suggested. — The  porpliyritical  crystals  formed  before  eruption 
must  have  sunk  to  the  bottom  of  the  fluid  mass,  for  the  specific  gravity  of 
hornblende  is  far  greater  than  the  mean  density  of  the  diorite,  and  the 
relation  can  hardly  have  been  reversed  at  the  temperature  at  which  they 
formed.  Little  as  we  know  of  the  subterranean  conditions  of  eruption,  it 
is  probably  safe  to  assume  that  the  upper  portion  of  a  fluid  or  plastic  mass 
would  be  extruded  before  the  lower,  and  that  the  portion  holding  the  por- 
phyritical crystals  in  suspension  would  be  the  last  to  appear.  The  dike  of 
porphyry  between  granitoid  walls  already  referred  to  seems  to  show  that 
this  was  the  case,  while  the  frequency  of  transitions  ig  evidence  that  the 
extrusion  was  a  nearly  continuous  process.  The  granular  groundmass  of 
the  porphyries  is  finer-grained  than  the  granitoid  rock,  but  this  does  not 
necessarily  prove  that  it  cooled  under  diff'erent  conditions,  for  a  certain  dif- 


OCCURRENCE  AND  SUCCESSION  OF  ROCKS.  195 

ference  in  chemical  composition  would  almost  inevitably  accompany  the 
supposed  separation  by  specific  gravity;  and  besides  the  porphyritical 
crystals,  other  more  minute  solid  particles  would  probably  also  sink,  and 
tend  to  the  multiplication  of  centei's  of  crystallization. 

Possibility  of  a  metamorphic  origin. — Whllc  the  evidcuccs  of  the  oruptivo  charac- 
ter of  this  diorite  are  tolerably  strong,  they  are  not  so  conclusive  as  to 
exclude  a  consideration  of  the  possibility  that  the  rock  may  be  metamorphic. 
As  has  been  shown  in  Chapter  III.,  one  variety  of  the  metamorphic  diorite 
is  almost  indistinguishable  per  se  from  the  rock  of  Mount  Davidson,  and 
another  variety  of  the  latter  is  distinctly  brecciated.  It  is  exceedingly  diffi- 
cult, if  it  is  not  in  the  present  state  of  knowledge  impossible,  to  comprehend 
how  the  formation  of  pure  and  shai-ply  developed  crystals  can  go  on  in 
media  not  svifficiently  mobile  to  be  regarded  as  fluid;  yet  we  know  that 
tourmalines,  garnets,  and  other  minerals  are  sometimes  beautifully  developed 
in  metamorphic  rocks,  which  have  not  only  retained  their  lamination,  but 
have  offered  an  efficient  resistance  to  the  pressure  of  thousands  of  feet  of 
overlying  strata.  Most  of  the  indications  of  the  eruptive  character  of  the 
Mount  Davidson  and  Cedar  Hill  diorite,  taken  singly,  are  thus  not  absolutely 
incompatible  with  a  metamorphic  origin.  But  until  the  origin  of  the  granitoid 
rocks  has  been  more  satisfactorily  elucidated  than  heretofore,  it  is  certainly 
the  duty  of  the  geologist,  while  giving  possible  alternatives  due  weight,  to 
judge  each  occurrence  on  its  own  merits,  and  to  seek  explanations  in  compre- 
hensible processes,  rather  than  through  unexplained  analogies.  At  present  an 
eruptive  origin  can  alone  be  regarded  as  probable  for  the  Washoe  diorites. 

Metamorphic  diorite. — Tlic  grouuds  for  cousideriug  the  metamorphic  diorite 
as  such,  have  already  been  given.  It  is  a  very  puzzling  rock  in  the  field, 
and  may  readily  be  mistaken  in  different  occurrences  for  granite,  diorite, 
augite-andesite,  or  basalt.  Wherever  the  underlying  rock  is  exposed  it  is 
sedimentary,  except  at  the  Wales  Consolidated,  where  the  metamorphism  has 
penetrated  to  the  underlying  granite.  It  is  also  associated  in  the  most  inti- 
mate way  with  the  quartz-porphyry,  and  does  not  appear  between  the 
stratified  rocks  and  eruptive  diorite.  If  the  area  occupied  by  the  quartz- 
porphyry  were  made  continuous,  it  would  completely  cover  all  the  meta- 
morphic diorite  in  the  District;  and  the  evidence  is  tolerably  strong  tliat 


196  GEOLOGY  OF  THE  COMSTOOK  LODfi. 

the  metamorphism  is  due  to  the  action  of  the  porphyry  on  the  strata  over 
which  it  flowed.  Metamorphic  diorite  occurs  on  the  Comstock  only  at  the 
extreme  south  end,  in  the  Silver  Hill  and  Justice  mines.  Mines  have  been 
sunk  in  it  south  of  Silver  City — for  example,  the  Amazon — and  have  struck 
ore  which  was  calcareous  and  carried  mixed  sulphurets.  The  Justice  ore 
associated  with  this  rock  was  of  a  similar  character. 

Quartz-porphyry. — The  quartz-porphyry  which  appears  on  the  map  is  merely 
the  northeasterly  corner  of  an  extensive  area  of  this  rock.  A  noticeable 
peculiarity  is  that  it  is  everywhere  decomposed,  and  everywhere  to  almost 
precisely  the  same  degree,  while  it  is  fissured  only  to  a  very  slight  extent. 
It  seems  scarcely  possible  that  this  decomposition  should  have  taken  place 
from  below,  for  the  underlying  granite  and  metamorphic  diorite  are  for  the 
most  part  very  fresh.  The  decomposition  would  seem  rather  the  result  of 
the  action  of  surface  waters,  favored  by  a  porous  structure.  This  structure 
is  perhaps  due  to  the  unequal  contraction  of  quartz  and  feldspar  in  cooling. 
Before  later  eruptions  covered  it,  the  porphyry  occupied  the  surface  for. a 
considerable  distance  farther  to  the  northeast  than  at  present,  for  it  appears 
in  the  Belcher  ground  and  in  the  Forman  shaft.  In  both  these  cases  it  under- 
lies hornblende-andesite,  while  in  the  Belcher  1648,  and  in  the  Overman,  it 
also  seems  to  underlie  diabase.  The  accessible  points  at  which  these  two 
rocks  come  in  contact,  however,  are  so  few  that  the  order  of  their  succession 
is  less  satisfactorily  made  out  than  that  of  any  other  important  members  of 
the  series  of  rocks  found  in  the  Washoe  District.  The  quartz-porphyry 
does  not  appear  to  be  intimately  associated  with  the  ore  bodies  of  the  Com- 
stock, though  occurring  near  to  some  of  those  in  the  Gold  Hill  mines ;  nor 
have  any  considerable  quantities  of  ore  been  discovered  in  this  rock  in  out- 
lying mines.  It  also  assays  little  or  nothing.  It  is  worthy  of  note  that 
quartz-porphyries  in  some  mining  districts  have  almost  certainly  supplied 
the  deposits  with  their  charge  of  precious  metals,  though  the  Washoe 
occurrence  is  so  barren. 

The  felsitic  modification  of  the  quartz-porphyry  is  confined  to  a  limited 
area  near  the  granite.  To  what  cause  the  difference  between  its  structure 
and  that  of  the  ordinary  variety  may  be  due  I  cannot  suggest. 


OCCUERENCE  AND  SUCCESSION  OF  ROCKS.  197 

Earlier  diabase. — The  diabases  are  almost  wholly  confined  to  the  mines,  only 
two  small  patches  having  been  discovered  on  the  surface.  Of  these,  that 
between  the  Julia  and  Ward  shafts  appears  normal  in  character  though  much 
altered,  and  as  it  occurs  at  the  bottom  of  a  ravine  vertically  above  the  main 
body  of  the  rock  nothing  is  easier  than  to  account  for  its  presence.  Such 
is  not  the  case  with  the  mass  in  Ophir  Ravine.  This  bears  a  very  strong 
outward  resemblance  to  a  granular  diorite,  and  it  seems  impossible  to  make 
out  a  sharp  contact  between  the  two  rocks.  There  is  also  no  evidence  of 
any  connection  between  this  area  and  the  main  mass  east  of  the  Lode.  As 
has  been  explained  in  Chapter  III.,  I  am  by  no  means  sure  that  it  should 
not  be  regarded  as  a  local  modification  of  diorite,  rather  than  an  independent 
eruption.  Apart  from  the  interest  attaching  to  such  an  occurrence  it  is  of 
little  importance,  no  further  consequences,  so  far  as  I  know,  depending  on 
its  determination.  As  may  be  seen  from  the  sections,  diabase  approaches 
the  surface  very  closely  immediately  below  the  city  of  Virginia,  so  closely 
that  at  least  a  few  croppings  would  be  expected  in  the  ground  covered  by 
the  town.  It  is  highly  probable  that  a  considerable  area  might  have  been 
traced  before  the  settlement  was  made,  but  the  ground  is  now  so  graded  and 
built  up  that  a  careful  search  failed  to  reveal  any  rock  in  place. 

Relations  to  the  Lode. — The  earlier  diabase  forms  the  east  or  hanging  wall  of 
the  Lode  throughout  its  more  productive  portion;  that  is,  from  the  Overman 
to  the  Sierra  Nevada,  and  from  the  surface,  or  very  close  to  it,  down  to  the 
lowest  depths  yet  reached.  It  also  penetrates  the  west  country,  at  the 
north  end  of  the  Lode,  in  stringers,  as  may  be  seen  on  the  horizontal  sec- 
tion on  the  Sutro  Tunnel  level.  Atlas-sheets  VIII.  and  IX.  This  fact  scarcely 
requires  explanation,  for  that  a  single  clean  fracture  of  the  diorite  mass 
should  have  been  effected  at  the  time  of  the  diabase  eruption  is  almost 
inconceivable.  If  the  diabase  succeeded  the  diorite  it  would  be  natural  to 
expect  diabase  in  fissures  within  the  diorite  masses,  and  fragments  of  diorite 
inclosed  in  diabase.  It  has  already  been  pointed  out  that  these  occur. 
There  is  a  considerable  sheet  of  diorite  east  of  the  bonanza  of  the  Califor- 
nia and  Consolidated  Virginia  mines,  and  similar  masses  were  encountered 
in  sinking  the  new  Yellow  Jacket  shaft.  In  the  higher  levels,  too,  it  is 
probable,  from  the  accounts  of  former  examinations,  that  diorite  horses  were 


198  GEOLOGY  OF  THE  GOMSTOCK  LODE. 

encountered.  On  this  point,  however,  there  is  some  uncertainty,  for  before 
the  identification  of  diabase  in  the  east  country  much  of  the  hanging  wall  now 
exposed  would  undoubtedly  have  been  recognized  as  an  older  rock  and 
confounded  with  diorite.  The  stringers  of  diabase  in  the  Sierra  Nevada  and 
the  Utah  mark  fissures  unquestionably  belonging  to  the  Comstock  system, 
and  that  in  the  former  mine  at  least  were  accompanied  by  a  very  trifling 
amount  of  ore.  The  history  of  the  Lode  and  the  chemical  discussions 
which  form  the  subjects  of  other  chapters,  make  it  highly  improbable 
that  bodies  of  any  consequence  will  ever  be  found  near  these  stringers. 
The  main  contact  of  the  diabase  with  the  diorites  swings  sharply  to  the 
northeast  in  the  Sierra  Nevada  ground,  and  has  not  been  explored  beyond 
that  point.  Diabase  does  not  appear  south  of  the  Overman,  and  the  Forman 
shaft  passed  from  hornblende-andesite  into  quartz-porphyry  at  2,200  feet 
from  the  surface.  If,  therefore,  as  there  is  reason  to  believe,  the  latter  rock 
preceded  the  diabase,  this  will  not  be  encountered  in  the  Forman  shaft.  The 
extension  of  the  diabase  in  an  easterly  direction  is  somewhat  uncertain.  On 
the  line  of  the  Sutro  Tunnel  the  diabase  is  only  about  1,300  feet  wide, 
measured  horizontally.  It  is  certainly  wider  than  this  at  the  Oshiston  shaft 
and  the  new  Yellow  Jacket.  The  Oshiston  is  beheved  to  have  met  diabase  at 
a  depth  of  about  1,000  feet,  though  the  locality  was  not  accessible,  while 
the  new  Yellow  Jacket  passed  into  it  at  less  than  400  feet  from  the  surface, 
indicating  an  extensive  body  still  farther  east. 

The  lithological  varieties  of  the  diabase  have  been  sufficiently  described 
in  a  former  chapter.  In  structure  it  resembles  the  diorite,  being  split  up 
near  the  Lode  into  rough  sheets  parallel  to  the  main  fissure,  as  has  been 
explained  in  Chapter  IV.  I  have  been  wholly  unable  to  see  any  evidence 
that  this  rock  was  not  emitted  at  a  single  outbreak.  Its  position,  lying  as  a 
mass  upon  a  diorite  wall  sloping  at  an  angle  of  about  45°,  together  with 
the  details  of  the  relations  of  the  two  rocks,  shows  that  it  is  younger  than 
the  dioiiites.  That  it  is  also  probably  younger  than  the  quartz-porphyry 
is  shown  by  the  occurrences  in  the  Overman,  which  are  not  fully  satisfactory 
only  because  they  are  so  limited. 

"Black  dike. — The  younger  diabase,  as  has  been  seen,  is  identical  with 
the  trap  of  New  Jersey.     It  has  often  been  confounded  with  the  black  slates 


OCCURRENCE  AND  SUCCESSION  OF  ROCKS.  199 

of  the  Gold  Hill  mines,  and  black  rocks  and  clays  have  sometimes  been 
classed  with  it  in  the  north  end  mines.  In  the  upper  levels  it  was  met  with 
only  in  an  indistinguishably  decomposed  form.  I  was  not  able  to  authen- 
ticate its  occurrence  north  of  the  Savage,  and  found  it,  wherever  struck,  of 
a  very  uniform  width,  always  a  few  feet,  never  more  than  a  couple  of  yards. 
From  the  Savage  to  the  Overman  it  generally  marks  the  contact  between  the 
older  diabase  and  the  west  wall  with  precision,  but  on  one  level  of  the  Chollar 
it  is  80  feet  west  of  the  contact,  and  in  the  Yelloiv  Jacket  a  narrow  belt  of 
slate  sometimes  lies  east  of  it.  In  the  Overman  the  dike  diverges  from  this 
contact,  extending  towards  American  Flat  as  far  as  the  Caledonia.  The  uni- 
form thickness  of  the  dike  shows  that  no  considerable  movement  between 
the  diabase  and  the  west  wall  took  place  at  or  previous  to  its  eruption,  for 
otherwise  the  fissure  which  it  filled  must  have  presented  the  enlargements 
and  contractions  characteristic  of  veins  the  walls  of  which  have  experienced 
a  relative  motion.  The  divergence  of  the  dike  towards  American  Flat  ex- 
plains the  so-called  forking  of  the  vein.  A  certain  amount  of  solfataric 
action  is  perceptible  along  the  dike  fissure,  accompanied  by  the  deposition 
of  quartz  which  is  not  wholly  barren  The  American  Flat  vein  is  a  stringer, 
the  position  of  which  was  predetermined  by  this  fissure. 

Earlier  hornbiende-andesite. — The  mine  workiugs  show  that  the  coutact  between 
the  earlier  diabase  and  the  earlier  hornbiende-andesite  is  very  steep,  and 
that  it  must  be  represented  by  a  line  something  like  that  indicated  in  the 
section  through  the  Sutro  Tunnel,  Atlas-sheet  VI.  The  inference  from  this 
section  is  strong  that  the  body  of  older  hornbiende-andesite  cut  by  the  tun- 
nel occupies  a  portion  at  least  of  the  fissure  through  which  it  was  erupted. 
The  eastern  surface  of  the  diabase  is  far  too  steep  to  admit  of  the  supposi- 
tion that  it  was  ever  exposed.  Previous  to  the  outbreak  of  the  hornbiende- 
andesite  the  diabase  must  either  have  extended  much  farther  east  than  now, 
or  a  mass  of  diorite  must  have  occupied  the  place  now  filled  by  hornbiende- 
andesite.  In  either  case,  the  rock  lying  east  of  the  present  limit  of  the 
diabase  must  have  been  submerged  by  the  andesite  eruption;  and  of  the 
two  suppositions  the  former  seems  the  more  probable.  The  augite-andesite 
stands  in  much  the  same  structural  relation  to  the  earlier  hornbiende-ande- 
site as  the  latter  holds  to  the  diabase,  and  the  east  and  west  surfaces  of  the 


200  GEOLOGY  OF  THE  GOMSTOCK  LODE. 

hornblende  rock,  as  seen  in  the  tunnel,  are  parallel  to  one  another  and  to 
the  Lode. 

Even  on  the  surface,  indications  of  the  parallelism  of  the  contact  be- 
tween the  two  andesites  and  the  Lode  are  observable.  If  a  line  is  drawn 
at  a  distance  of  4,500  feet  east  of  the  vein,  it  will  fall  very  close  to  the 
easternmost  edges  of  the  earlier  hornblende-andesite,  and  include  only  one 
considerable  tract  of  the  aug-ite  rock  between  it  and  the  Lode.  The  For- 
man  shaft  is  nearly  at  the  center  of  this  tract,  and  the  section  through  it 
shows  that  hornblende-andesite  exists  below  the  surface.  The  contact  be- 
tween these  two  rocks  in  depth  is  therefore  probably  nearly  parallel  to  the 
Lode  throughout  the  whole  length  of  the  latter. 

Besides  the  area  of  earlier  hornblende-andesite  to  the  east  of  the  Lode, 
it  covers  a  large  extent  of  country  to  the  west  of  the  diorite.  Before  the 
fault  occurred  the  top  of  the  range  was  probably  about  on  a  level  with  the 
east  wall,  and  it  seems  probable  that  the  whole  exposure  of  earlier  horn- 
blende-andesite is  ascribable  to  a  single  eruption,  or  an  unbroken  series  of 
eruptions.  I  can  find  'no  indication  of  bedding,  nor  of  the  distinct  lava 
streams  which  give  evidence  of  intermittent  action  in  the  neighborhood  of 
modern  volcanoes.  At  first  the  andesite  most  likely  buried  the  diorite  com- 
pletely, but  the  latter  must  have  been  reexposed  by  erosion  before  the 
fault  took  place.  The  hornblende-andesite,  as  well  as  the  diabase,  is  di- 
vided into  sheets  by  a  system  of  parallel  fissures.  If  the  conclusions  drawn 
in  Chapter  IV.  are  correct,  this  fissure  system  was  developed  by  faulting  a1 
a  comparatively  recent  period,  but  the  tendency  to  parallelism  in  the  struct- 
ure of  the  country  was  first  exhibited  as  far  back  as  the  earher  hornblende- 
andesite  eruption. 

The  area  north  of  Silver  City  is  remarkable  for  the  unusual  develop- 
ment of  hornblende  crystals,  which  are  frequently  an  inch  and  a  half  in 
length,  and  occasionally  more.  In  this  area,  too,  there  are  several  sharp 
cones  one  or  two  hundred  feet  in  height,  which  suggest  volcanic  vents,  but 
no  craters  are  traceable ;  and  the  evidence  of  degradation  opposite  Virginia, 
especially  the  flatness  of  the  surface  previous  to  the  fault,  as  is  proved  from 
the  present  regular  character  of  the  fault-curve,  makes  it  improbable  that 
distinguishable  relics  of  craters  or  cones  of  eruption  should  remain.     In  the 


OCCUERENCE  AJSTD  SUCCESSION  OF  ROCKS.  201 

area  north  of  Cedar  Hill  Canon  this  andesite  is  much  less  homogeneous 
than  usual,  varying  in  texture  from  coarse  to  fine  frequently,  and  almost 
without  transitions.  These  differences  have  been  emphasized  by  decom- 
position and  erosion,  which  have  carved  out  projecting-  dike-like  sheets, 
fantastic  columns,  and  the  like,  from  the  heterogeneous  mass. 

That  this  rock  is  younger  than  the  quartz-porphyry  and  the  diabase  is 
very  evident  from  the  sections,  since  it  overlies  these  rocks  vertically  in 
wide  areas,  while  there  is  nothing  in  their  relations  suggesting  laccolitic 
masses. 

There  are  some  east-and-west  veins  in  the  hornblende-andesite  near 
Silver  City,  which  are  said  to  have  yielded  in  the  aggregate  considerable 
quantities  of  bullion.  The  only  mines  which  could  have  thrown  any  light 
on  the  origin  of  this  ore,  however,  were  closed  at  the  time  of  the  examina- 
tion. They  are  near  the  Justice  mine,  which  shows  a  great  complication  of 
rocks  in  its  ore-bearing  region,  and  the  ore  of  the  east-and-west  veins  is 
very  probably  due  to  the  same  general  causes  as  the  Justice  ore-body.  The 
andesites  themselves  do  not  give  considerable  assays. 

Augite-andesite. — lu  its  genei'al  features,  the  occurrence  of  augite-andesite 
closely  resembles  that  of  the  preceding  rock.  It,  too,  appears  to  have  issued 
on  a  fissure  nearly  parallel  to  the  Lode  and  to  have  spread  very  extensively 
over  the  country;  indeed,  the  present  surface  shows  a  greater  area  of  it 
than  of  the  earlier  hornblende-andesite.  It  is  possible  that  its  eruptions 
were  not  confined  to  the  fissure  cut  by  the  Sutro  Tunnel.  Basalt  Hill,  B  6, 
for  example,  still  some  300  feet  high,  may  well  'be  a  relic  of  a  still  larger 
eruptive  cone,  rather  than  a  remnant  of  an  overflow  from  a  fissure  at  a  con- 
siderable distance.  Like  the  older  andesite  the  relations  of  the  augite  rock 
to  the  faulted  surface  near  Virginia  seem  to  show  that  it  was  eroded  down  to 
a  level  in  that  region  before  the  fault  occurred.  Its  character  throughout 
the  District  is,  as  a  rule,  very  uniform.  It  is  possible,  however,  that  a 
few  localities  described  as  hornblende-andesite  are  in  reality  local  modi- 
fications of  this  rock.  Thus  the  rock  containing  the  hornblendes  with 
two  concentric  belts  of  magnetite,  a  crystal  from  which  is  shown  in  Fig.  17, 
Plate  III.,  is  exposed  only  by  a  cut  1,000  feet  east  of  the  railroad  station,  in 
C  7.     It  is  within  but  very  near  the  edge  of  an  area  of  augite-andesite, 


202  GEOLOGY  OF  THE  COMSTOCK  LODE. 

which  appears  everywhere  to  lie  directly  upon  qiiartz-porphyry  or  still 
older  rocks.  If  hornblende-andesite  proper  occurs  here,  it  should  show  at 
the  contacts;  but  the  nearest  area  of  the  hornblende  rock  is  6,000  feet 
away.  If  this  is  properly  to  be  classed  with  the  augite-andesites  in  spite 
of  its  mineralogical  composition,  it  is  quite  possible  that  the  three  small 
patches  of  earlier  hornblende-andesite  shown  on  the  map,  each  of  them 
entirely  surrounded  by  augite-andesite,  may  also  be  of  this  character. 

Independence  of  the  augite-andesite  eruption. The   tWO  rOCks  arC   SO    mUch  alike  that 

some  hthologists  doubt  the  propriety  of  classifying  them  as  different  species, 
but  in  the  Washoe  District  they  are  certainly  diflPerent  eruptions.  The 
contacts  in  the  Sutro  Tunnel,  the  Forman  shaft,  and  at  many  points  on  the 
surface  are  well  defined,  and  the  mineralogical  character  is  persistent  over 
very  large  areas,  in  spite  of  a  few  doubtful  localities.  It  has  been  seen  that 
there  are  also  points  where  it  is  very  difficult  to  say  whether  the  rock  is  to 
be  regarded  as  diorite  or  diabase.  The  absence  of  such  occurrences  would 
be  a  matter  of  surprise,  for  the  character  of  a  rock  depends  upon  combina- 
tions of  chemical  and  physical  conditions,  which  cannot  be  identical  at  any 
two  points.  Each  so-called  rock  species  represents  an  endless  number  of 
such  combinations,  and  some  of  these  are  indistinguishable  from  those  at- 
tending the  formation  of  allied  species.  The  strange  fact  is  not  the  occur- 
rence of  transitions,  which  are  after  all  exceptional,  but  the  persistence  of 
rock  types  not  only  within  limited  areas  but  throughout  the  world. 

In  determining  the  succession  of  the  hornblende  and  augite-andesites 
position  alone  can  be  relied  upon,  for  the  two  rocks  are  so  closely  alhed 
that  it  would  be  impossible  to  distinguish  with  certainty  between  an  inclu- 
sion and  a  local  modification  in  composition.  The  indications  of  position, 
however,  all  tend  to  the  supposition  that  the  hornblendic  rock  is  the  older, 
as  may  be  seen  from  an  inspection  of  the  sections. 

Occidental  lode. — The  Occideutal  lode  occurs  in  augite-andesite.  Unfortu- 
nately the  principal  mines  were  closed  at  the  period  of  the  investigation, 
and  it  could  not  be  studied  satisfactorily.  The  dump  of  the  Occidental  mine 
seems  to  show  that  a  contact  with  micaceous  diorite  is  encountered  in  the 
workings.  This  lode  is  plotted  on  the  map  from  distinct  croppings  and 
mine  surveys,  and  its  trace  is  a  further  remarkable  illustration  of  the  par- 


OCCURRENCE  AND  SUCCESSION  OF  ROCKS.  203 

allelism  of  structure  so  frequently  referred  to.  Even  the  sinuous  form  of 
the  CoMSTOCK  is  almost  exactly  reproduced  in  the  Occidental  lode.  Bedded 
flows  aggregating  over  a  mile  in  thickness  could  never  have  resulted  in  so 
nearly  perfect  a  parallelism. 

Later  hornbiende-andesite. — The  Sutfo  Tunficl  sectiou  sliows  a  fourth  vcry  steep 
contact  between  augite-audesite  and  younger  hornbiende-andesite ;  but  the 
eastern  portion  of  the  former,  though  covered  for  the  most  part,  did  not 
sink  in  the  younger  rock  below  the  level  of  the  tunnel,  and  even  reaches 
the  surface  near  the  mouth  of  the  adit.  The  manner  in  which  the  portions 
of  diabase  and  earlier  hornbiende-andesite  which  lay  to  the  east  of  the 
masses  now  in  place  disappeared,  is  a  matter  of  speculation;  the  Sutro 
Tunnel  section  shows  that  the  corresponding  area  of  augite-andesite  really 
sank  into  the  later  hornbiende-andesite.  Had  it  settled  a  few  hundred  feet 
farther,  it  would  have  left  as  little  trace  behind  it  as  did  the  earlier  rocks. 
So  far  as  the  Washoe  District  is  concerned,  however,  the  eruption  of  later 
hornbiende-andesite  was  probably  less  violent  and  less  voluminous  than 
that  of  either  of  the  preceding  andesites,  and  was  therefore  not  so  likely  to 
bury  the  east  country  to  a  great  depth.  Above  ground,  instead  of  lying 
on  a  curved  surface  reducible  to  an  original  plain,  it  forms  a  range  of 
mountains  extending  to  the  north  far  beyond  the  limits  of  the  map.  These 
do  not  appear  to  have  suffered  greatly  from  erosion,  for  even  near  the  sum- 
mits they  are  largely  composed  of  tufa  and  tufaceous  breccia,  which  could 
off'er  little  resistance  to  water  currents.  It  does  not  appear  to  me  that  the 
existence  of  this  range  in  its  present  form  is  compatible  with  the  suppo- 
sition that  a  large  area  of  the  same  rock  has  been  removed  by  erosion. 
Making  allowance  for  faulting,  the  older  and  firmer  rocks  have  been  worn 
down  to  a  tolerably  smooth  and  uniform  surface,  upon  which  the  present 
younger  hornbiende-andesite  range  lies  in  rugged  masses.  Had  the  older 
andesites  and  the  diorite  been  cut  away  after  the  formation  of  these  hills,  the 
latter  must  have  suffered  at  least  as  much  as  the  older  rocks. 

Evidence  of  slight  erosion. — Thd'c  is  uo  evidcucc  that  they  have  done  so;  on 
the  contrary,  if  the  contours  of  the  map  within  the  area  laid  down  as  younger 
hornbiende-andesite  are  examined,  it  will  be  seen  that  these  are  not  such  as 
commonly  result  from  deep  erosion.     Compare,  for  example,  the  steep  slopes 


204  GEOLOGY  OF  THE  COMSTOCK  LODE. 

of  the  Howery  range  with  the  older  hornblende-andesite  declivity  west  of 
Ophir  Hill.  In  the  latter  locality  every  water-way  has  eaten  deeply  into 
the  rock,  and  every  slightest  undulation  in  the  line  of  cliffs  has  given  rise 
to  an  eroding  streamlet  during  wet  weather.  On  the  Flowery  range  the 
drainage  channels  are  far  apart,  and  very  shallow,  and  many  undulations 
which  in  a  deeply  eroded  district  would  be  sure  to  be  emphasized  by  water 
carving  show  nothing  of  the  sort.  The  contact  line  between  this  rock  and 
the  augite-andesite  seems  to  me  unlike  contacts  developed  by  erosion.  It 
has  a  very  different  character  from  the  other  contacts  in  the  District,  and 
reminds  one  strongly  of  the  forms  assumed  by  slag  slowly  oozing  over  the 
floor  of  a  smelting-works.  The  structure  of  the  rock,  as  seen  on  large 
exposures,  appears  to  indicate  subaerial  rather  than  subterranean  deposition. 
Plate  VII.  shows  the  east  flank  of  Mount  Rose,  and  is  accurately  repro- 
duced from  a  photograph.  Rude,  thick  layers  of  eruptive  material,  mostly 
tufa  and  breccia,  are  plainly  visible  in  this  locality,  though  they  are  trace- 
able over  no  great  distance.  It  is  easy  to  see  how  such  beds  might  form 
in  successive  eruptions,  or  through  the  variations  in  activity  of  a  single  pro- 
longed eruption;  but  it  is  difficult  to  account  for  such  a  structure  in  a  mass 
which  has  cooled  beneath  the  surface,  and  has  been  exposgd  by  erosion. 
Such  a  mass  would  be  characterized  by  dike-structure  rather  than  by  beds. 
The  physical  character  of  the  varieties  of  this  rock,  considered  with 
reference  to  their  occurrence,  is  also  difficult  to  reconcile  with  the  suppo- 
sition that  the  range  is  a  mere  relic  of  erosion.  As  has  been  explained,  in 
Chapter  III.,  some  of  the  younger  hornblende-andesite  is  dense  and  glassy, 
and  other  modifications  are  firm  enough  to  resist  decomposition  better 
than  ordinary  augite-andesite.  In  an  eroded  district  these  harder  rocks 
would  be  looked  for  on  the  summit,  and  the  soft  tufas  would  be  found,  if 
at  all,  in  protected  localities;  but,  as  has  been  pointed  out,  the  tufas  are  most 
abundant  at  the  summits.  Deeply  eroded  areas  of  eruptive  rocks  almost 
always  show  patches  isolated,  or  patches  nearly  separated  from  the  main 
field,  by  the  action  of  water.  To  a  certain  extent  this  is  the  case  with  the 
younger  hornblende-andesite,  for  the  two  little  areas  near  the  Sierra  Nevada 
mine  were  unquestionably  cut  off  from  the  tongue  of  this  rock  extending 
from  the  Flowery  range  towards  the  Utah,  by  the  erosion  of  Seven  Mile 


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OCCUEEENCE  AND  SUCCESSION  OF  EOCKS.  205 

Canon.  Mount  Abbie,  on  the  other  hand,  shows  amphitheatrical  basins, 
which  are  not  impossibly  relics  of  craters,  and  that  mountain  very  likely 
represents  a  separate,  though  unimportant,  eruption.  But  had  tlie  rock 
covered  much  more  country  than  at  present,  it  is  almost  certain  that  other 
patches  would  have  been  cut  off  exactly  as  those  near  the  Sierra  Nevada 
have  been,  and  as  various  tracts  of  augite-andesite,  quartz-porphyry,  etc., 
have  been  separated  from  one  another.  It  has  been  supposed  that  there 
were  such  patches  near  the  Combination  shaft  and  the  new  Yellow  Jacket, 
but  the  statements  rest  upon  erroneous  determinations. 

In  the  Sutro  Tunnel  the  fissure  system  parallel  to  the  Lode  extends  to 
the  younger  hornblende-andesite,  but  though  this  rock,  particularly  near  its 
western  limit,  shows  evidences  of  dynamical  action,  I  was  not  able  to  make 
certain  of  any  regular  partings  within  its  mass.  On  mere  geometrical 
grounds  it  could  hardly  be  expected  that  the  fissures  would  be  traceable  in 
this  rock,  for  at  so  great  a  distance  from  the  Lode  the  logarithmic  curve 
and  its  asymptote  sensibly  coincide. 

There  can  be  no  doubt  that  the  younger  hornblende-andesite  succeeded 
the  augite-andesite.  In  the  Sutro  Tunnel  section  it  is  seen  directly  over- 
lying and  inclosing  the  augitic  rock,  and  on  the  divide  between  Mount  Kate 
and  Mount  Rose  the  augite-andesite  can  be  traced  passing  horizontally  be- 
neath the  trachytic-looking  porphyry.  The  peninsular-like  area  near  Sutro 
Shaft  III.  would  seem,  too,  to  be  a  flow  from  the  main  body,  not  an  inde- 
pendent or  subsidiary  eruption;  for  the  tunnel,  though  passing  close  by  this 
area,  shows  none  of  the  younger  rock  west  of  Shaft  II.,  nor  is  there  any 
sign  of  special  disturbance  of  the  augite-andesite  in  the  tunnel  near  Shaft 
III. 

Basalt. — Besides  the  five  little  patches  of  basalt  shown  on  the  map, 
there  is  another  of  about  the  same  size  directly  west  of  these,  and  just  be- 
yond the  limits  of  the  map.  It  is  said  that  a  few  miles  farther  south  there 
are  considerable  areas  of  this  rock.  Two  of  the  five  occurrences  shown  are 
very  characteristic  mesas,  and  the  rock  is  in  every  way  typical.  The  only 
remarkable  fact  connected  with  it  is  its  small  extension.  No  general  eff'ect  • 
upon  the  history  of  the  Disteict  has  been  certainly  traced  to  it.  Though  the 
basalt  comes  in  contact  only  with  pre-Tertiary  rocks  and  earlier  hornblende- 


206  GEOLOGY  OF  THE  COMSTOCK  LODE. 

andesite,  there  can  be  little  doubt  that  it  is  the  youngest  of  all.  Its  relations  to 
the  andesites  have  been  observed  in  a  great  number  of  localities  in  the  western 
United  States,  and  it  has  always  been  found  to  succeed  them.  This  general 
evidence  is  strengthened  in  the  present  case  by  the  extreme  freshness  of  the 
olivine,  which  even  under  the  microscope  often  shows  no  trace  of  decom- 
position. As  olivine  is  the  most  readily  decomposed  of  all  the  lithologically 
important  minerals,  this  fact  is  evidence  that  the  basalt  is  very  recent. 

Period  of  soifatarism. — The  gcologists  who  havc  studlcd  the  CoMSTOCK  have 
always  sought  to  connect  the  solfataric  action,  which  is  so  important  a  feat- 
ure of  the  District,  with  one  pr  other  of  the  volcanic  eruptions.  Since  the 
augite-andesite  and  the  rocks  which  preceded  it  are  deeply  altered  by  soi- 
fatarism, and  even  portions  of  the  younger  hornblende-andesite  are  also  thus 
affected,  the  general  decomposition  cannot  be  placed  earlier  than  the  erup- 
tion of  the  last-mentioned  rock.  Portions  of  an  eruptive  rock  may  be  im- 
mediately decomposed  by  the  emanations  accompanying  its  ejection,  but 
before  an  extensive  area  can  be  decomposed  throughout,  it  must  probably 
cool  and  be  shattered  by  mechanical  action  sufficiently  to  admit  a  some- 
what free  penetration  of  active  solutions.  If  the  solfataric  action  is  due  to 
one  of  the  eruptions,  it  must  then  be  either  to  that  of  the  younger  horn- 
blende-andesite or  to  that  of  the  basalt.  But  direct  evidence  of  such  a 
connection  is  wanting.  The  focal  line  of  soifatarism  is  at  or  close  to  the 
Lode.  The  younger  hornblende-andesite  area  shows  no  trace  of  it  except 
where  it  approaches  the  vein ;  and,  as  has  been  mentioned,  the  basalt  shows 
no  effects  of  solfataric  decomposition.  It  is  also  somewhat  difficult  to  under- 
stand how  an  eruption  can  produce  extraordinarily  intense  solfataric  action 
at  a  locality  somewhat  remote  from  the  vent  of  its  own  fluid  ejecta,  and  not 
also  at  or  close  to  that  vent ;  though  I  by  no  means  deny  the  possibility 
of  such  a  coincidence. 

While,  ho.wever,  the  solfataric  action  appears  to  me,  beyond  question, 
one  of  the  series  of  volcanic  events  of  which  the  history  of  the  District 
is  so  full,  it  does  not  seem  to  be  necessarily  connected  immediately  with  an 
eruption  of  lava.  There  are  mud  volcanoes;  and  solfataras  are  often  active 
at  periods  of  time  remote  from  those  of  eruptions  in  their  neighborhood, 
and  though  the  emission  of  heated  watei's  frequently  attends  igneous  erup- 


OCOUERElSrCB  AND  SUCCESSION  OF  ROCKS.  207 

tions,  thei'e  appears  no  reason  to  suppose  that  vast  quantities  of  heated 
fluids  may  not  be  driven  to  the  surface  without  an  accompaniment  of  lava. 
The  solfataric  action  and  the  fault  are  certainly  contemporaneous,  and  may 
together  form  the  entire  volcanic  manifestation  of  the  period  in  which  they 
occurred.  If  the3^  were  independent  of  the  eruption  of  younger  horn- 
blende-andesite,  they  must  have  been  subsequent  to  it;  and  I  beheve  it 
most  probable  that  such  was  the  case.  Of  their  time  relations  to  the  basalt 
eruption  there  is  no  means  of  judging. 

Collections. — The  disputcd  character  of  a  number  of  the  rocks  of  the 
District  made  verj^  full  collections  essential  to  the  substantiation  of  the 
views  maintained  in  this  report.  A  cabinet  series  of  200  specimens  was 
collected  in  tripHcate,  one  set  being  designed  for  the  hthological  collection 
of  the  National  Museum,  a  second  for  the  geographical  collection  of  the 
same  institution,  and  a  third  for  the  San  Francisco  office  of  the  Geological 
Survey.  By  order  of  the  Director,  the  size  of  these  specimens  is  4  inches 
by  5  inches,  their  thickness  being  from  an  inch  to  an  inch  and  a  half 
Though  these  specimens,  selected  with  a  view  to  representing  the  District 
as  well  as  possible,  amply  suffice  for  the  ordinary  purposes  of  study,  so  small 
a  number  was  not  found  sufficient  to  justify  the  geological  map  and  sections. 
A  working  collection  without  duplicates  was  therefore  also  gathered.  The 
size  adopted  was  only  1 1  by  2^  inches,  in  order  to  lessen  the  labor  of 
gathering  them  and  to  facilitate  their  use  in  the  office.  This  collection  con- 
tains over  2,000  numbers.  Slides  were  ground  whenever  they  seemed  likely 
to  afford  desirable  information,  and  the  total  number  cut  was  about  600. 

The  locality  of  every  specimen  was  recorded  at  the  time  of  collection 
on  a  map  of  the  surface  or  of  the  mines  as  the  case  might  be.  The  mine 
maps  employed  were  on  a  large  scale,  and  the  localities  are  usually  accurate 
to  three  or  four  feet.  The  surface  map  being  on  a  comparatively  small  scale 
the  positions  are  less  precise,  but  are  recorded  as  accurately  as  practicable. 
On  the  sections  of  the  Lode,  shown  in  the  Atlas,  the  points  from  which  speci- 
mens were  collected  are  marked  by  crosses,  while  each  locality  from  which 
there  is  a  slide  is  indicated  by  a  large  black  dot.  On  the  surface  map  a 
red  cross  shows  the  localities  microscopically  determined,  a  single  cross 


208  GEOLOGY  OP  THE  COMSTOCK  LODE. 

often  representing  a  number  of  slides.  Accompanying  the  collections  is  a 
copy  of  the  surface  map,  showing  the  position  of  each  specimen  with  its 
number.  The  numbers  of  specimens  of  which  there  are  slides  are  under- 
lined, and  cabinet  specimens  are  distinguished  from  those  of  the  working 
collection.  The  collections  are  also  fully  labeled  and  completely  cata- 
logued. 


CHAPTER   VI. 
CHEMISTRY. 

General  nature  of  the  chemical  activity. The    general    TCSVlltS   of    chemical    aCtlvity 

whicli  have  been  observed  in  the  Washoe  District  can  be  very  briefly 
stated.  Decomposition  is  widespread,  but  while  in  the  greater  part  of  the 
area  it  has  not  sei'iously  modified  the  character  of  the  rock,  the  alteration 
within  a  certain  portion  of  the  region  is  profound,  and  often  wholly  obscures 
lithological  distinctions.  This  area  of  extreme  decomposition  is  precisely 
the  most  important,  lying  immediately  about  the  Lode.-^  The  characteristic 
bisilicates  of  the  eruptive  rocks  have  been  replaced  by  chloritic  minerals, 
epidote,  quartz,  and  calcite;  pyrite  has  been  deposited  in  the  mass  of  the 
rock,  and  the  feldspars  have  in  great  part  undergone  degeneration  of  a 
complex  kind;  finally,  oi-e-bearing  quartz  has  been  deposited  in  the  Lode. 
It  is  the  purpose  of  the  present  chapter  to  give  as  rational  an  account  of 
these  changes  as  I  am  able  to  suggest,  and  to  trace  their  geological  rela- 
tions. Any  such  account  must,  in  the  present  state  of  knowledge  as  to 
the  constitution  of  minerals,  be  lai-gely  hypothetical;  but,  although  future 
investigations  will  probably  greatly  modify  the  present  conceptions  of  the 
nature  of  inorganic  compounds,  the  best  hypotheses  at  present  are  those 
which  put  the  least  strain  on  well-proved  theories.  The  Washoe  District 
affords,  as  has  been  seen,  a  remarkable  opportunity  for  microscopic  exam- 
ination of  the  results  of  decomposition ;  not,  however,  for  their  chemical 
investigation,  for  no  occurrences  have  been  met  with  in  which  single  alter- 
ation products,  excepting  pyrite,  are  concentrated  in  sufficient  masses  to  fur- 
nish good  material  for  analysis.     Even  were  this  the  case,  it  seems  to  me  that 

'  See  Fig.  1. 
14  0  L,  209 


210  GEOLOGY  OF  THE  COMSTOCK  LODE. 

little  progress  can  be  made  until  definite  criteria  are  discovered  by  which 
the  state  of  combination  of  the  elements  in  fresh  minerals  can  be  decided. 
Formation  of  pyrite. — Pcrliaps  tlio  most  stHking  charactcrlstic  of  the  decom- 
posed rocks  of  Washoe  is  the  presence  of  innumerable  bright  crystals  of 
pyrite  disseminated  through  the  mass.  The  unaltered  rocks  do  not  appear 
to  carry  this  mineral;  if  it  occurs  in  them  at  all  it  is  certainly  a  very  rare 
ingredient.  In  the  altered  rocks  pyrite,  when  present,  is  abiindant  nearly  in 
proportion  to  the  degree  of  decomposition,  and,  except  where  it  is  exposed 
to  the  direct  action  of  the  atmosphere,  it  is  almost  invariably  perfectly  fresh. 
All  the  circumstances  thus  indicate  that  it  is  a  product  of  decomposition. 
The  massive  rocks  contain  iron,  chiefly  as  magnetite  and  as  a  component  of 
the  bisilicates;  and  the  pyrite  must  have  been  formed  by  the  action  of  soluble 
sulphurets  on  one  or  both  of  these  compounds.  The  slides  of  the  pyritous 
,  rocks,  however,  frequently  show  large  quantities  of  sharply  defined  mag- 
netite, while  the  bisilicates  are  in  a  majority  of  cases  wholly  decomposed. 
There  is  certainly  nothing  in  the  association  of  pyrite  and  magnetite  to  sug- 
gest a  relation ;  but  the  pseudomorphs  of  decomposition  products  after  the 
bisilicates  are  very  frequently  studded  with  small  pyrite  crystals,  and  occa- 
sionally real  pseudomorphs  of  pyrite  after  augite  or  hornblende  appear  to 
occur.  Of  these  it  is  difiicult  to  be  certain,' however;  for  the  size  of  the 
pyrite  individuals  is  usually  considerable,  relatively  to  that  of  their  hosts;  the 
original  crystal  form  is  consequently  never  unmodified,  and  is  commonly 
altered  beyond  recognition.  The  distribution  of  the  pyrite  in  the  rock  also 
reminds  the  familiar  observer  of  the  distribution  of  the  bisilicates  in  the 
same  rock,  and  macroscopical  comparison  of  suites  of  specimens  from  the 
same  localities  shows  that  the  pyrite  to  all  appearances  is  associated  with 
the  bisilicates,  and  in  extreme  cases  replaces  them.  It  is  easy  to  lay  too 
much  stress  on  an  impression  of  this  sort,  yet  when  such  an  impression  is 
derived  from  the  examination  of  many  thousand  instances  it  deserves  some 
weight.  All  the  evidence  thus  tends  to  the  supposition  that  the  pyrite  is 
mainly  a  decomposition  product  of  the  bisilicates  and  of  mica.  Such  an 
alteration  is  quite  possible  in  the  presence  of  alkaline  sulphides,  or  of 
hydrosulphuric  acid ;  and,  as  has  been  seen,  the  waters  even  now  entering 
the  mines  three  thousand  feet  from  the  surface  are  charged  with  the  latter 


CHEMISTRY.  211 

reagent.  Had  oxidizing  agencies  been  active  to  any  great  extent  below  the 
surface  tlie  pyrite  must  have  been  decomposed,  and  the  inference  from  the 
facts  is  strong  that  such  has  not  been  the  case. 

The  formation  of  pyrite  might  conceivably  either  take  place  immedi- 
ately at  the  expense  of  the  bisilicates  or  be  formed  from  secondary  minerals; 
biit  the  ferruginous  silicates,  chlorite  and  epidote,  are  frequently  deposited 
in  veins  and  patches  quite  free  from  pyrite,  and  nothing  has  been  observed 
in  their  association  with  pyi*ite  to  indicate  an  epigenetic  connection.  It  is 
therefore  more  probable  that  p^aite  resulted  immediately  from  the  action  of 
hydrosulphuric  acid  and  similar  compounds  on  the  bisilicates.  This  action 
could  not  possibly  be  unaccompanied  by  the  formation  of  other  alteration 
pi'oducts,  for  the  whole  stochiometric  relations  of  the  bisilicates  would  be 
changed  by  the  abstraction  of  iron.  Since  hydrosulphuric  acid  Is  a  pow- 
erful reducing  agent,  it  Is  a  priori  probable  that  the  accompanying  products 
would  contain  little  ferric  oxide,  and  as  the  bases  in  the  bisilicates  are  fully 
saturated  with  silicon  a  separation  of  silicic  acid  is  indicated. 

Formation  of  chlorite. — The  clilorite,  whIch,  as  has  been  seen  in  Chapter  III., 
nearly  always  results  from  the  decomposition  of  the  ferro-magnesian  sili- 
cates, is  of  uncertain  species,  but  it  Is  neither  cllnochlore  nor  pennine,  and 
answers  well  to  Werner's  chlorite  (the  ripidollte  of  Gr.  Rose).  This  mineral 
has  approximately  the  composition  of  a  semisilicate,  and  contains  little  or 
no  ferric  oxide.  It  is  also  accompanied  In  a  great  proportion  of  cases  by 
secondary  quartz,  and  often  also  by  calclte.  The  occurrence  of  this  last 
mineral  shows  that  carbonic  acid,  as  well  as  hydrogen  sulphide,  must  have 
been  present  during  the  decomposition  of  the  rocks,  and  probably  fronf  the 
commencement,  for  chlorite  contains  no  calcium;  and  had  hydrosulphuric 
acid  alone  acted  on  the  bisilicates  a  calcium  silicate  m^jst  have  resulted  in 
the  first  instance.  Of  such  a  preliminary  change,  however,  there  is  no 
trace,  although,  as  has  been  seen  in  Chapter  III.,  it  appears  possible  to  fol- 
low the  course  of  decomposition  mineralogically  from  its  Incipient  stages. 
Calcite,  however,  Is  not  usually  prominent  among  the  decomposition  pro- 
ducts of  the  bisilicates  in  specimens  collected  under  ground,  unquestionably 
owing  to  its  great  solubility. 

Circumstances  favoring  the  formation  of  epidote. That      chloritC     Or     clllorltlc     Uliuerals 


212  GEOLOGY  OF  THE  COMSTOCK  LODE. 

very  usually  result  from  the  decomposition  of  hornblende,  augite,  and  mica 
is  a  well-known  fact,  and  pseudomorphs  of  epidote,  after  these  minerals,  are 
also  common.  The  difference  is  great,  for  while  chlorite  contains  little  or 
no  ferric  oxide  and  no  calcium,  epidote  contains  both,  but  is  free  from  mag- 
nesium. It  would  seem,  therefore,  as  if  epidote  must  be  formed  under 
such  conditions  that  fen-ous  compounds  might  be  oxidized,  or  such  that 
ferric  compounds,  at  all  events,  would  not  be  reduced,  and,  further,  under 
conditions  favoring  the  solubility  of  magnesian  salts  rather  than  those  of 
calcium.  It  appears  to  me  somewhat  difficult  to  suppose  the  bisilicates 
exposed  to  a  sulphidizing  action  so  strong  as  to  result  in  the  formation  of 
pyrite,  and  yet  not  sufficiently  reducing  to  prevent  the  formation  of  ferric 
compounds.  On  the  other  hand,  pyrite,  though  of  very  variable  stability, 
often  oxidizes  with  great  difficulty,  and  the  oxidation  of  ferrous  compounds 
may  sometimes  be  effected  in  its  presence.  Epidote  might,  therefore,  form 
in  the  presence  of  pyrite,  but  hardly  contemporaneously  with  it.  The 
behavior  of  the  salts  of  magnesium  and  calcium  salts  towards  one  another 
is  known  to  var}^  greatl}^  with  the  physical  conditions,  especially  with  tem- 
perature, and  presumably  also  with  pressure,  and  it  is  further  affected  by 
the  concentration  of  solutions.  Thus,  Dr.  T.  S.  Hunt^  found  that  when  solu- 
tions of  the  chloi'ides  and  carbonates  of  these  elements  are  evaporated  at 
ordinary  temperatures,  calcium  carbonate  alone  is  first  precipitated;  while, 
when  the  solution  is  boiled,  magnesium  carbonate  first  separates.  This  and 
similar  facts  tend  to  the  supposition  that  high  temperatures  would  favor  the 
formation  of  magnesian  chlorite  rather  than  of  calciferous  epidote. 

'conditions  under  which  epidote  occurs. — The  undcrgrouud  rocks  at  Washoe  all  con- 
tain chlorite  in  abundance,  but  epidote  is  uncommon.  Thus,  a  special  search 
was  necessary  to  discover  epidote  in  the  underground  diabases,  while  per- 
haps half  the  augite-andesites  from  the  surface  contain  it  in  considerable 
quantities.  When  it  occurs  at  a  considerable  depth  it  seems  to  be  either 
close  to  the  Lode  or  near  strong  seams  extending  towards  the  surface,  as  in 
one  or  two  localities  in  the  Sutro  Tunnel.  On  the  surface  epidote  is  extremely 
common,  tinging  whole  areas  of  the  various  rocks  with  its  peculiar  green, 
and  occurring  in  many  and  widely  separated  localities.     Where  epidote  is 

'  Chem.  and  Geolog.  Essays,  p.  138. 


CHEMISTRY.  213 

best  developed,  as  in  Crown  Point  and  Ophir  ravines,  the  accompanying 
pyrite  is  usually  decomposed  either  wholly  or  in  part;  and  in  localities  at  a 
small  distance  beneath  the  surface,  like  the  McKibben  tunnel,  it  is  in  those 
belts  of  rock  which  are  evidently  most  highly  decomposed  that  epidote  is 
found  replacing  chlorite. 

Probable  course  of  the  alteration  of  chlorite  to  epidote. StrOHg    minOralogical     evldeUCe 

has  already  been  offered  to  show  that  epidote  at  Washoe  is  an  alteration 
product  of  chlorite.  The  indications  of  relative  solubility  are  worth  con- 
sidering in  this  connection.  Chlorite  is  manifestly  rather  easily  soluble,  and 
soon  after  its  formation  becomes  diffused  through  the  groundmass  and  any 
porous  crystals  which  may  be  present,  settling,  too,  in  veins  when  cracks  offer 
an  opportunity  for  such  a  concentration.  Epidote  appears  to  be  soluble  only 
in  a  greatly  inferior  degree ;  indeed,  its  faggot-like  masses  of  crystals  seldom 
show  anything  which  can  be  interpreted  as  attack  by  a  solvent.  If,  there- 
fore, chlorite  and  solutions  of  calcium  carbonate  containing  free  oxygen  are 
brought  together  under  physical  conditions  compatible  with  the  formation 
of  epidote,  it  seems  inevitable  that  epidote  should  be  precipitated,  unless 
still  more  insoluble  substances  may  also  be  thrown  down  under  the  same 
conditions. 

It  is  well  known  that  chlorite  is  frequently  altered  to  a  mass  of  quartz, 
ferric  hydrate,  and  carbonates.  When  this  change  takes  place  it  is  prob- 
able that  at  least  a  portion  of  the  alumina  is  mingled  in  some  form  with  the 
iron  oxide,  and  the  carbonates  most  likely  contain  magnesium  as  well  as 
calcium.  In  the  numerous  cases  of  this  change  which  have  been  observed 
at  Washoe,  the  carbonates  form  a  large  portion  of  the  resulting  mixture,  a 
fact  which  appears  to  prove  that  the  active  solutions  were  but  slightly  charged 
with  carbonic  acid,  since,  had  it  been  otherwise,  calcite  and  magnesite,  if 
separated  out  at  all,  would  have  been  redissolved.  Cases  of  the  conversion 
of  chlorite  to  epidote  and  to  carbonates,  etc.,  often  occur  in  the  same  slide, 
and  presumably  under  nearly  the  same  physical  conditions.  It  may  be 
that  the  decisive  point  is  the  quantity  of  carbonic  acid  present.  If  the  two 
processes  went  on  at  different  times  such  a  difference  would  be  readily  expli- 
cable, and  if  simultaneously  it  is  not  difficult  to  understand  how  the  quantity 
of  carbonic  acid  might  vary.    Though  rocks  are  permeable,  the  aqueous  cur- 


214  GEOLOGY  OF  THE  COMSTOCK  LODE. 

rents  are  greatly  obstructed,  and  move  in  labyrinthine  paths  of  least  resist- 
ance. Of  this  the  lithologist  is  constantly  reminded  by  meeting  wholly  fresh 
crystals  and  entirely  decomposed  ones  of  the  same  mineral  close  together. 
One  tiny  current  percolating  through  the  rock  may  meet  with  comparatively 
large  quantities  of  carbonates  and  become  saturated,  while  another  in  the 
same  neighborhood  remains  well  charged  with  carbonic  acid  and  oxygen. 
If  the  suggestion  made  is  correct,  the  former  coming  in  contact  with  chlo- 
rite would  convert  it  into  a  mass  of  carbonates,  quartz,  and  ferric  oxide; 
while  the  latter,  which  would  be  a  solvent  for  carbonates,  would  convert 
chlorite  into  epidote. 

Nature  of  the  decomposition  of  the  bisiiicatcs. — Qualified  by  all  tho  doubts  wliich  have 
been  expressed,  the  observations  considered  in  connection  with  the  chemical 
possibilities  lead  to  the  following  as  the  most  probable  statement  of  the 
decomposition  of  the  bisilicates  of  the  Washoe  rocks.  Waters  charged 
with  hydrosulphuric  and  carbonic  acids,  but  containing  no  free  oxygen,  at 
temperatures  probably  very  near  the  boiling-point,  acted  upon  the  fresh 
augite  and  hornblende  ('or  mica),  producing  from  them  pyrite,  chlorite, 
quartz,  and  carbonates  of  the  alkaline  earths  simultaneously  Of  these  a 
large  portion  of  the  carbonates  passed  into  solution.  At  a  later  period  sur- 
face waters  at  lower  temperatures,  containing  carbonic  acid  and  free  oxygen 
in  solution,  produced  a  further  alteration  of  a  portion  of  the  chlorite  in  the 
rocks  near  the  surface,  or  peculiarly  accessible  from  it.  Where  carbonic 
acid  was  present  in  excess  epidote  resulted ;  where,  through  saturation  with 
carbonates,  the  carbonic  acid  was  deficient,  the  chlorite  was  altered  to  car- 
bonates, quartz,  and  metallic  oxides,  no  doubt  with  admixtures  of  less  impor- 
tant compounds. 

Magnetite. — No  place  has  been  given  to  magnetite  among  the  decomposi- 
tion prodvicts  of  the  bisilicates.  As  all  the  rocks  contain  large  quantities 
of  this  mineral  constantly  associated  with  the  bisilicates,  and  often  so  thickly 
distributed  in  perfectly  fresh  crystals  (particularly  of  hornblende)  as  to  leave 
but  little  of  the  host  visible,  it  is  difficult  to  distinguish  sharply  between 
the  primitive  and  the  secondary  occurrences  of  the  iron  ore.  In  fact,  I  have 
not  been  able  to  make  absolutely  sure  of  more  than  one  or  two  instances 
of  secondary  magnetite,  though  such  an  origin  seems  probable  enough  in 


CHEMISTRY.  215 

many  cases.  On  the  other  hand,  it  seems  certain  that  the  black  border  of 
many  hornblendes  has  been  attacked,  and  has  given  place  to  a  transparent 
mineral,  which  is  more  or  less  diffused  in  and  obscured  by  the  groundmass. 
The  natural  supposition  is  that  it  is  ferrous  carbonate. 

iimenite. — Titauic  iron  ore  may  often  be  observed  in  slides  from  the  Dis- 
trict passing  into  leucoxene.  The  nature  of  this  substance  is  doubtful, 
and  no  occurrence  in  the  District  is  conclusive  as  to  its  nature,  yet  many 
cases  have  been  observed  the  character  of  which  would  be  very  satisfactorily 
accounted  for  if  the  supposition  of  Messrs.  Fouquti  &  L^vy,  that  leucoxene 
and  titanite  are  identical,  were  accepted. 

Decomposition  of  the  feldspars. — The  feldspars  of  tlic  Washoe  regiou  have  offered 
a  far  more  effectual  resistance  to  decomposing  agencies  than  the  bisilicates, 
much  more,  too,  than  would  be  supposed  from  a  macroscopical  examination 
of  the  rocks.  In  the  mines  it  is  very  rarely  that  a  particle  of  auo-ite, 
hornblende,  or  mica,  can  be  found,  these  minerals  being  nearly  always 
wholly  replaced  by  alteration  products;  but  it  is  the  exception  when  a 
moderately  hard  rock  does  not  show  under  the  microscope  well  defined  and 
fairly  fresh  feldspars.  When  wholly  unattacked  the  feldspars  of  diabase, 
of  some  diorites,  and  of  the  older  andesites  are  transparent,  and  the  rocks 
then  show  only  the  tints  due  to  the  presence  of  magnetite  and  the  bisilicates. 
They  are  then  dark,  somewhat  basaltic-looking  masses.  But  when  only  a 
very  minute  amount  of  change  has  taken  place  in  the  feldspars,  they  become 
opaque  through  irregular  reflection,  and  form  the  most  prominent  feature 
of  the  rock.  Rough  estimates,  made  with  the  help  of  the  microscope,  indi- 
cate that  the  decomposition  of  much  less  than  one  per  cent,  of  the  feldspar 
substance  suffices  to  destroy  the  transparency  of  the  crystals. 

The  nature  of  the  decomposition  of  the  feldspars  is  still  very  obscure. 
It  is  usually  considered  that  the  triclinic  feldspars  as  well  as  orthoclase  are 
sometimes  converted  into  kaolin,  though  Professor  Tschermak  maintains, 
as  an  analytical  result,  that  the  hydrated  aluminium  silicate  resulting  from 
the  alteration  of  plagioclase  contains  but  a  single  molecule  of  water,  and 
not  two,  as  is  the  case  with  kaolin.  Saussurite  and  pinitoid  are  the  names 
given  to  complex  silicates,  or  mixtures  of  silicates  and  other  substances, 


216  GEOLOGY  OF  THE  COMSTOCK  LODE. 

which  often  result  from  the  decomposition  of  feldspars;  and  mica  and  epidote 
are  counted  among  the  pi'oducts  of  alteration. 

Kaolin. — Kaolin  is  microscopically  an  obscure  mineral.  According  to 
Mr.  H.  Fischer  it  is  amorphous,  while  Mr.  A.  Knop  found  it  to  consist  of 
delicate  hexagonal  plates  of  the  rhombic  system.  Breithaupt  named  this 
crystaUine  modification  nacrite,  and  M.  Des  Cloizeaux  pholerite.  If  saussurite 
and  pinitoid  are  really  independent  minerals,  it  is  certain  that  these  names 
have  also  been  given  to  mere  mixtures  resulting  from  the  extraction  of  por- 
tions of  the  silicic  acid  and  of  the  stronger  bases. 

Evidence  of  the  microscope. — lu  tlic  Washoe  rocks,  as  is  usual  elscwherc,  the 
first  indication  of  decomposition  is  the  appearance  of  calcite  and  quartz  in 
the  more  or  less  carious  crystals.  This  is  doubtless  attended  by  the  forma- 
tion of  soluble  alkaline  silicates,  which,  however,  are  not  recognizable  under 
the  microscope.  As  the  process  continues  the  striations  are  obliterated,  and 
the  final  result  is  a  heterogeneous  mass  showing  aggregate  polarization, 
sometimes  only  faintly  translucent,  and  containing  in  a  recognizable  form 
only  grains  of  calcite  and  quartz.  No  amorphous  substance  has  been  ob- 
served, nor  any  hexagonal  lamellae  answering  to  the  description  of  nacrite. 
Mica,  too,  appears  to  be  absent,  although  occurring  among  the  decomposi- 
tion products  of  similar  rocks  at  no  great  distance  from  Virginia.  Chlorite 
and  epidote  are  common  in  decomposed  feldspars,  but  in  many  cases  it  seems 
certain  that  chlorite  due  to  the  decomposition  of  the  bisilicates  has  merely 
permeated  the  spongy  mass;  and  epidote  has  repeatedly  been  observed 
developing  in  patches  of  chlorite,  which  were  surrounded  by  feldspar  sub- 
stances, just  as  it  has  been  described  and  illustrated  as  occurring  in  altered 
bisilicates.  No  case  has  been  met  with  in  which  either  mineral  was  dis- 
tinctly parasitic  on  feldspar. 

All  lithologists  agree  that  chlorite  forms  from  the  bisilicates,  and  that 
feldspars  become  carious;  it  is  also  acknowledged  that  chlorite  is  diffused 
through  the  portions  of  the  rock  mass  in  the  immediate  neighborhood  of 
the  point  at  which  it  forms.  It  must  therefore  penetrate  the  feldspars  where 
these  are  partially  decomposed,  in  all  rocks  in  which  the  bisilicates  are  to 
any  extent  converted  into  chlorite.  It  is,  of  course,  by  no  means  necessary 
that  the  point  at  which  chlorite  gained  access  to  the  feldspar  should  be 


CHEMISTRY.  217 

visible,  for  entrance  is  as  likely  to  have  been  effected  above  or  below  the 
plane  of  a  thin  section  as  in  it.  If  chlorite  and  epidote  really  occur  as 
results  of  the  decomposition  of  feldspar,  it  should  be  easy  to  show  the 
parasitic  growth  of  chlorite  in  feldspars,  just  as  its  development  from  horn- 
blende has  been  shown  in  the  present  volume. 

Chemical  analysis. — The  microscope  glvcs  mainly  negative  results  concerning 
the  decomposition  of  the  feldspars  of  the  Washoe  rocks.  Chemical  analysis 
of  the  decomposition  products  could  lead  to  no  definite  results,  because  no 
reasonably  pure  material  could  be  obtained,  and  the  only  remaining  source 
of  information  is  the  analysis  of  the  rocks.  The  diabase  from  the  hangino- 
wall  of  the  Lode,  which  was  analyzed,  is  a  very  slightly  altered  rock,  and 
has  been  described  under  slide  18.  Its  feldspars  are  transparent  and  have 
undergone  only  an  inappreciable  amount  of  alteration ;  the  rock  nevertheless 
contains  a  considerable  quantity  of  water,  as  is  shown  by  its  loss  in  ignition, 
2.47  per  cent.  Abundant  fluid  inckisions  account  for  a  part  of  this  loss, 
and  the  water  of  hydration  of  the  small  amount  of  chlorite  it  contains  for 
another  portion.  The  ignition  loss  no  doubt  includes  a  small  amount  of 
carbonic  acid.  The  "propylite  horse"  analyzed  by  Prof  W.  G.  Mixter  was 
in  all  probability  decomposed  diabase.  An  inspection  of  the  analysis  shows 
either  that  silica  had  been  deposited  in  the  rock,  or  what  seems  more  likely, 
that  the  bases  had  been  in  large  part  extracted.  It  contained  1.83  per  cent, 
of  water,  or  about  two-thirds  as  much  as  the  fresh  rock.  The  bisilicates 
must  have  been  represented  by  chlorite,  which  contains  about  12  per  cent, 
of  water.  The  small  quantity  of  aluminium  not  entering  into  the  chlorite 
may  possibly  have  existed  as  kaolin,  a  supposition  neither  proved  nor  dis- 
proved by  the  analysis,  which,  however,  shows  that  the  horse  contained  at 
most  a  small  percentage  of  that  mineral.  Four  analyses  of  clays  made  for 
the  Exploration  of  the  Fortieth  Parallel"  by  Professors  Johnson  and  Mixter 
are  available.  It  is  here,  if  anywhere,  that  kaolin  must  be  indicated.  On 
comparison  of  these  analyses  with  that  of  the  fresh  diabase,  it  appears  that 
they  do  not  represent  concentrations  of  any  special  mineral,  but  merely 
highly  altered  rock  masses.  Barring  the  pyrite  and  water,  the  first  three 
show  very  nearly  the  same  composition  as  the  fresh  rock,  while  a  portion 
of  the  silicic  acid  has  apparently  been  abstracted  from  the  Savage  clay. 


218  GEOLOGY  OF  THE  COMSTOCK  LODE. 

The  quantity  of  pyrite  corresponds  fairly  well  with  the  deficiency  of  iron 
in  the  clays.  Taking  into  consideration  that  these  clays  must  have  con- 
tained chlorite  corresponding  to  about  18  per  cent,  of  augite,  it  appears 
from  the  water  contents  that  those  fi'om  the  Chollar  and  the  Hale  <£  Nor- 
cross  can  have  included  little  or  no  kaolin.  Those  from  the  Yellow  Jacket 
and  the  Savage,  on  the  other  hand,  may  have  contained  both  chlorite  and 
kaolin,  but  the  latter  only  to  the  extent  of  a  few  per  cent. 

Kaolinization  not  prevalent  at  Washoe. Tho  Weight  of  CvideUCe  iS  thuS  TCaSOnably 

strong  that  in  the  regions  thus  far  exploited  on  and  near  the  Comstock, 
kaolinization,  if  it  has  taken  place  at  all,  has  occurred  only  to  a  very  trifling 
extent,  and  that  the  degeneration  of  the  feldspars  results  almost  wholly  in  a 
mixture  of  silica,  calcite,  and  unrecognizable  minerals,  earthy  in  texture,  in 
part  nearly  opaque,  and  of  a  light  color. 

Occurrence  of  ore  and  the  accompanying  rocks. As  may  be    SeeU  ft'Om   the    mapS  aud 

sections,  the  Comstock  Lode  is  several  miles  long,  and  is  found  in  contact 
with  various  rocks.  The  fissure  is  not  simple,  but  ramified,  and  might  have 
been  represented  as  still  more  complex,  for  the  quartz  veins  struck  by  the 
McKibben  Tunnel  in  Spanish  Ravine,  and  by  the  Peytona  and  other  work- 
ings on  Cedar  Hill,  are  unquestionably  either  stringers  joining  the  Lode  at 
unknown  points,  or  subsidiary  parallel  veins  due  to  the  same  chain  of  dy- 
namical and  chemical  causes  as  the  Comstock.  It  appears  from  the  longi- 
tudinal vertical  projection  that  but  a  small  fraction  of  the  fissure  has  been 
filled  with  ore.  This  statement,  however,  requires  explanation  and  qualifi- 
cation. Nearly  all  the  vast  mass  of  quartz  on  the  Comstock  contains  con- 
siderable quantities  of  silver  and  gold,  but  none,  of  course,  is  extracted 
which  will  not  pay  for  working.  While  auriferous  gravels  may  yield  a 
handsome  profit  when  they  contain  considerably  less  than  ten  cents  per  ton, 
and  gold  quartz  may  sometimes  pay  which  contains  two  or  three  dollars, 
Comstock  ores  carrying  less  than  about  twenty  dollars  can  usually  be 
extracted  only  at  a  loss.  Geologically  the  Comstock  must  be  considered 
as  filled  with  metalliferous  gangue,  enriched  at  numerous  spots,  which  are 
known  by  the  Spanish  mining  term  "bonanzas." 

Vastly  the  most  productive  area  has  been  that  portion  of  the  main 
Lode  between  the  Overman  and  the  south  end  of  the  Sierra  Nevada  mine. 


CHEMISTEY.  219 

Bullion  has  also  been  produced  at  the  Jws^ice  to  the  south,  and  from  the 
veins  on  Cedar  Hill  to  the  north.  In  the  Virginia  and  Gold  Hill  mines, 
and  on  Cedar  Hill,  the  gangue  is  quartz,  only  occasional  masses  of  calcite 
of  insignificant  size  having  been  encountered.  South  of  the  Overman,  on 
the  other  hand,  the  gangue  is  largely  calcite. 

The  quartz  of  Cedar  Hill  carries  free  gold,  alloyed,  of  course,  with  a 
little  silver.  Certain  stringers  from  the  main  Lode  and  the  "west  vein"  of 
the  CoMSTOCK,  as  that  portion  lying  to  the  west  of  the  great  horse  in  Vir- 
ginia City,  above  the  line  at  which  the  two  fissures  join,  is  usually  called, 
are  of  the  same  character.  The  Justice  ore  was  argentiferous,  but  very 
"base,"  carrying  large  quantities  of  galena,  zinc  blende,  etc.  The  ore 
bodies  on  the  main  Lode  in  Virginia  and  Gold  Hill,  which  have  yielded 
almost  all  of  the  bullion  extracted,  may  profitably  be  considered  as  of  two 
classes.  The  greater  portion  of  the  bullion  has  been  derived  from  minerals 
disseminated  in  the  quartz  in  microscopic  particles.  Ore  of  this  kind  is 
often  distinguishable  from  barren  quartz  by  bluish  stains,  but  not  always. 
The  quality,  and  even  the  presence  of  ore,  can  in  many  cases  only  be  told 
by  assay,  and  superintendents  who  have  taken  part  in  the  mining  opera- 
tions almost  from  their  commencement  do  not  hesitate  to  confess  that  their 
judgment  of  the  quartz  is  often  at  fault.  The  behavior  of  this  ore  in  amal- 
gamation shows  that  its  silver  contents  is  mainly  due  to  argentite.  Its  gold 
contents  constitutes  from  one-quarter  to  one-half  its  total  value.  Near  the 
outcroppings  many  bunches  of  other  ores  occurred,  such  as  stephanite, 
polybasite,  ruby  silver,  etc.  These  were  in  some  cases  accompanied  by 
relatively  large  quantities  of  galena  and  zinc  blende.  In  the  great  Consol- 
idated Virginia  and  California  bonanza,  several  streaks  or  veins  of  very 
rich  black  silver  ores,  said  to  be  mainly  stephanite,  occurred.  These  were 
sepai-ated  from  the  surrounding  ore-bearing  quartz  very  sharply,  as  if  of 
later  origin. 

Pyrite  is  found  everywhere,  both  in  the  country  rock  and  in  the  ore 
disseminated  in  small  crystals.  It  is  less  frequent  in  the  quartz  than  in  the 
country  rock,  but  it  is  especially  abundant  in  the  east  country,  opposite  the 
ore  bodies.  It  also  occurs  with  frequency  in  the  diorite  west  of  and  near 
the  Lode.     In  all  these  cases  it  forms  but  a  small  portion  of  the  mass — say 


220  GEOLOGY  OF  THE  COMSTOCK  LODE. 

from  ten  per  cent,  downwards;  but  in  the  graphitic  slates  forming  the  west 
wall  in  the  Gold  Hill  mines,  bunches  are  met  with  in  which  it  is  the  pre- 
dominant constituent.  These  are,  however,  usually  only  a  cubic  foot  or 
two  in  size,  and  appear  to  occur  only  close  to  the  vein.  As  a  rule,  the  slates 
are  not  much  more  pyritiferous  than  the  diabase. 

Relations  between  ores  and  rocks. — There  is  au  cvideut  relatiou  between  the  in- 
closing rocks  and  the  character  of  the  ore.  The  rocks  occurring  at  and 
near  the  Justice,  with  its  refractory  ores  and  calcite  gangue,  are  metamor- 
phic  diorite,  mica-diorite,  quartz-porphyry,  and  hornblende-andesite.  The 
Cedar  Hill  gold-quartz  veins  are  in  diorite:  The  ores  of  the  more  impor- 
tant mines  lie  on  the  contact  between  diabase  and  diorite. 

There  seem  to  be  but  two  probable  ways  in  which  these  differences 
can  have  come  about.^  The  ore  deposits  might  have  taken  place  at  differ- 
ent times,  and  therefore  under  different  conditions,  or  the  contents  of  the 
fissures  may  have  been  extracted  from  their  walls  at  the  same  time,  and  the 
differences  be  due  to  the  composition  of  the  siirrounding  rock.  If  the 
Cedar  Hill  veins  were  deposited  at  a  different  time  from  the  main  mass  of 
the  CoMSTOCK  ore,  it  must  have  been  at  an  earlier  date,  for  the  vast  quan^ 
titles  of  solutions  which  reached  the  Comstock  could  not  have  failed  to 
penetrate  the  fissured  diorite.  Not  only  stringers  from  the  Comstock,  how- 
ever, but  even  the  "west  vein,"  are  of  the  same  character  as  the  Cedar  Hill 
quartz.  When  this  west  quartz  was  deposited  the  fissure  below  was  cer- 
tainly open,  and  had  it  been  deposited  before  the  argentiferous  ore,  it  is 
scarcely  possible  to  suppose  that  it  would  not  also  have  filled  the  vein  at 
lower  points.  If  they  were  to  be  assigned  to  different  periods,  one  would 
also  expect  to  find  either  gold  veins  in  the  east  country,  or  silver  veins  in  the 
west.  In  short,  there  is  much  to  show  that  these  two  classes  of  deposits 
were  contemporaneous;  and  I  know  of  no  evidence  tending  to  show  that 
they  are  not  ascribable  to  a  single  period.  The  Justice  ore  body  is  not 
closely  enough  connected  with  the  more  important  portion  of  the  Com- 

'  It  is  also  conceivable  that  the  ores  should  have  been  precipitated  from  solution  by  the  rock 
forming  the  ■walls  and  the  horses,  and  that  the  observed  diflferences  are  due  to  the  character  of  the 
precipitant.  All  the  evidence  of  ore  deposits  in  general,  and  of  the  Comstock  in  particular,  however, 
appear  to  me  to  point  to  changes  of  temperature  and  pressure,  evaporation  and  the  action  of  liquid 
reagents,  as  the  causes  of  precipitation.  In  describing  the  Lode  I  shall  be  obliged  to  recur  to  this 
subject. 


CHEMISTRY.  221 

STOCK  to  permit  of  a  detailed  comparison,  such  as  that  given  above,  but 
in  the  absence  of  proof  to  the  contrary  it  is  probable  that  it  too  was  depos- 
ited at  the  same  time. 

Time  relations  of  the  ore. — During  the  poHod  in  which  the  field  work  for  the 
present  volume  was  done,  there  was  but  very  little  ore  in  sight.  What  I  have 
seen  of  ore  near  the  croppings  exposed  in  a  few  reopened  workings,  however, 
and  recollections  of  the  streaks  of  high-grade  ore  in  the  "great  bonanza,"  lead 
to  the  belief  that  these  rich  concentrations  were  of  later  origin  than  the  mass 
of  the  ore.  The  quartz  in  the  Consolidated  Virginia  and  California  was 
almost  everywhere  a  crushed,  powdery  mass,  while  the  thin  and  persistent 
veins  of  black  ore  running  through  it  wei'e  very  solid.  A  somewhat  simi- 
lar relation  seems  to  have  existed  near  the  croppings,  and  it  is  not  impossi- 
ble that  these  ores  were  formed  at  the  expense  of  others  of  the  more  usual 
kind  at  a  later  date,  and  that  they  occupy  spaces  opened  in  the  ore  masses 
bv  faulting  action. 

Origin  of  the  vein  minerals. — It  is  wcll  kuowu  that  the  able  aud  laborious  inves- 
tigations of  Prof.  F.  Sandberger^  have  added  greatly  to  our  knowledge  of 
the  distribution  of  the  metals  in  unaltered  rocks,  and  of  the  reactions  by 
which  in  many  cases  they  have  been  concentrated  in  veins.  Though  not  the 
first  to  show  that  the  bisilicates,  as  well  as  mica,  sometimes  carry  small 
quantities  of  the  heavy  metals,  he  has  multiplied  the  known  instances  so 
greatly  as  to  establish  the  frequency  of  such  a  composition.  In  many 
cases  it  is  an  exceedingly  complex  matter  to  prove  a  possible  connection 
between  a  vein  and  the  surrounding  rock,  because  the  minerals  present  in 
noticeable  quantities  are  numerous.  This  is  not  the  case  at  Washoe,  for 
quartz,  silver,  gold,  and  sulphur  predominate  so  greatly  over  all  other  ele- 
ments that  if  the  presence  of  these  is  accounted  for,  the  problem  may  be 
considered  solved,  unless  the  solution  offered  is  inconsistent  with  the  pres- 
ence of  small  quantities  of  calcite,  galena,  zinc  blende,  etc.,  and  with  the 
general  distribution  of  pyrite. 

Origin  of  the  quartz  and  ore. — No  chcmical  aualysis  is  uccessary  to  detect  a 
possible  origin  for  the  quartz  of  the  Lode.  Macroscopical  and  microscop- 
ical examinations  sufficiently  show  the  enormous  destruction  of  primary  sili- 

'  Untersuchungeu  iiber  Erzgange,  erstes  Heft,  1882.     Also,  Berg-  n.  h.-Zeit.ung,  1877  and  1880. 


222  GEOLOGY  OF  THE  COMSTOCK  LODE. 

cates  which  has  taken  place  throughout  a  large  area.  On  the  other  hand, 
minute  quantities  of  gold  and  silver  can  be  more  easily  and  more  certainly 
determined  bj^  dry  assay  than  by  analysis,  provided  that  pure  lead  reagents 
can  be  procui'ed  But  the  selection  of  suitable  material  for  the  investiga- 
tion of  the  gold  and  silver  contents  of  the  Washoe  rocks  was  by  no  means 
a  simple  matter.  As  has  been  seen,  there  is  but  one  spot  known  in  which 
nearly  fresh  diabase  can  be  collected,  and  that  close  to  the  Comstock  fissure. 
Moreover,  the  quantities  of  the  precious  metals  to  be  dealt  with  are  so  minute 
that  a  mere  trace  of  infiltrating  solutions  of  their  compounds  would  impart 
a  comparatively  important  metallic  contents,  and  that  such  impregnations 
occur  in  some  of  the  rocks  there  is  very  good  reason  to  believe.  This 
occurrence  of  fresh  diabase  is  therefore  open  to  suspicion.  If,  however,  the 
diabase  which  forms  the  east  or  hanging  wall  of  the  Lode  is  the  source  of 
its  gold  and  silver,  fresh  portions  of  the  rock  will  show  a  larger  quantity  of 
the  precious  metals  than  decomposed  samples;  while,  if  the  source  of  the 
ore  were  independent  of  the  diabase,  decomposed  portions  of  the  latter, 
being  more  porous,  would  have  been  more  readily  and  fully  impregnated 
by  the  metalliferous  solutions.  Moreover,  it  has  been  shown  that  pyrite 
forms  at  the  expense  of  the  augite  of  the  diabase,  and  as  pyrite  is  known  to 
have  a  very  strong  affinity  for  gold,  the  decomposed  pyritiferous  rock  should 
show  a  greater  proportion  of  gold  to  silver  than  the  fresh  diabase,  if  this  rock 
is  the  source  of  the  metals.  Were  the  original  distribution  of  gold  and  silver 
and  their  subsequent  extraction  nearly  uniform,  the  composition  of  the  ore  in 
the  Lode  would  correspond  to  the  contents  of  the  fresh  rock,  less  that  of  the 
decomposed  rock  and  the  pyrite,  as  shown  by  a  limited  number  of  assays. 
The  quantity  of  the  precious  metals  occurring  in  the  vein  should  also  be 
calculable  from  the  extent  of  the  decomposed  rock.  Such  ideal  conditions, 
however,  are  not  to  be  expected.  The  excessive  difficulty  of  obtaining  a 
representative  sample  of  any  gold  or  silver  deposit  is  familiar  to  all  mining 
men,  and  in  the  Comstock  itself  great  variations,  both  in  the  relations  of 
gold  to  silver  and  in  the  total  tenor,  are  of  constant  occurrence.  On  the 
supposition  that  the  metals  have  been  extracted  from  the  diabase  these 
variations  indicate  great  irregularity  in  the  leaching  action  or  in  the  original 
distribution  of  the  metals,  or,  more  probably,  in  both. 


CHEMISTRY.  223 

Precautions  observed  in  assaying. — The  assavs  tabulated  at  the  end  of  Chapter  III. 
were  made  by  my  assistant,  Mr.  J.  S.  Curtis,  who,  in  addition  to  a  thorough 
training,  has  had  many  years  of  experience  in  accurate  and  responsible 
assaying.  In  attempting  to  detect  minute  quantities  of  precious  metals  in 
the  Washoe  rocks,  the  first  difficulty  experienced  was  in  obtaining  suffi 
ciently  2:»ure  lead  or  litharge  It  was  found  that  even  that  imported  fi-om 
Germany  and  sold  at  a  very  high  price  as  chemically  pure  was  far  too  rich 
in  silver  and  too  irregular  in  its  silvei-  contents  to  answer  the  purpose.  In 
this  dilemma  Mr.  Rickard,of  the  Richmond  Mining  and  Smelting  Company,  in 
Eureka,  was  kind  enoiigh  to  place  a  refining  furnace  with  a  new  test  at  Mr. 
Curtis's  disposal,  as  well  as  the  purest  of  the  lead  refined  by  the  Luce  & 
Kozan  process  in  the  works  under  his  charge.  By  careful  manipulation  Mr. 
Curtis  was  able  to  prepare  litharge  assaying  less  than  eight  cents  a  ton  and 
of  so  regular  a  composition  that,  with  the  help  of  blank  assays,  the  silver 
contents  of  the  rocks  could  be  very  exactly  determined. 

A  series  of  experiments  was  then  made  to  determine  the  time  of  reduc- 
tion which  would  give  a  maximum  result  with  material  so  poor  in  metals 
as  the  Washoe  rocks.  It  was  found  that  this  time  was  much  longer  than 
that  requisite  for  the  reduction  of  ore.  Refined  cream  of  tartar  was  the 
reducing  agent  employed,  with  sodium  bicarbonate  and  borax  in  carefully 
determined  proportions  as  fluxes.  The  cupels  were  made  with  great  care 
of  two  parts  of  bone-ash  to  one  of  cedar-ash,  the  surface  being  formed  of 
elutriated  bone-ash.  In  cupelling  feather-litharge  was  invariably  allowed 
to  form,  and  throughout  the  experiments  no  known  pi-ecaution  was  neg- 
lected. 

Gold  detected  in  the  rocks. — Li  addition  to  the  sllver  contents  of  the  Washoe 
rocks,  gold  also  was  detected,  but  in  such  minute  quantities  that  little  reliance 
can  be  placed  upon  the  relative  tenor  of  diffei'ent  samples.  It  was  estab- 
lished, however,  that  the  fresh  diabase  cari-ies  as  much  as  four  or  five  cents  in 
gold  to  the  ton,  and  furthermore  that  the  pyrite,  so  abundant  in  the  decom- 
posed rocks,  carries  both  gold  and  silver,  but  more  of  the  former  than  of  the 
latter.  Thus  pyrite  washed  from  the  decomposed  diabase  250  feet  north  of 
the  C.  c(-  C.  connection  with  the  North  Lateral  of  the  8utro  Tunnel,  assayed 
three  cents  in  silver  and  eight  cents  in  gold,  and  pyrite  from  the  Belcher 


224  GEOLOGY  OF  THE  COMSTOCK  LODE. 

slates  gave  eighteen  cents  silver  and  twenty  cents  gold.  The  diorite  from 
Bullion  Ravine  also  showed  an  indeterminably  small  trace  of  gold,  while 
the  andesites  carry  about  as  much  as  the  diabase. 

Silver  traced  to  the  augite. — It  sccmed  probablc  from  Professor  Sandberger's 
investigations  that  the  augite  of  the  diabase  was  the  seat  of  its  metallic  con- 
tents. To  test  this  point,  the  feldspar  and  augite  were  separated  by  Thoulet's 
method  and  separately  assayed.  It  appeared  that,  for  equal  weights,  the 
augite  was  eight  times  as  rich  as  the  feldspathic  material,  and,  as  a  per- 
fectly clean  separation  by  Thoulet's  method  is  impracticable,  this  seems 
substantially  equivalent  to  a  proof  that  the  silver  is  a  constituent  of  the 
augite. 

Results  of  the  assays. — By  comparisou  of  thc  diiferent  assays  it  appears  that 
decomposed  diabase  carries  somewhat  less  than  half  as  much  silver  as  the 
fresh  rock.  Where  the  decomposed  rocks  are  pyritous,  the  experiments 
made  do  not  indicate  any  essential  diminution  of  the  gold  contents.  This 
fact,  however,  is  quite  possibly  due  to  irregularity  in  distribution  and  the 
minuteness  of  the  quantities  of  gold  to  be  determined.  As  the  decomposi- 
tion of  the  rock  in  question  has  proceeded  at  a  great  depth  beneath  the  sur- 
face, it  is  highly  unlikel}^  that  silver  should  have  been  extracted  unaccom- 
panied by  gold.  Much  of  the  decomposed  rock,  too,  is  nearly  free  from 
pyrite,  and  had  the  gold  contents  of  such  specimens  been  determined  a 
smaller  percentage  would  probably  have  been  found.  The  omission  was 
not  detected  until  too  late  to  resume  the  investigation.  So  far  as  quantita- 
tive relations  are  concerned,  only  the  silver  can  be  relied  on,  though  the 
qualitative  detection  of  gold  as  well  is  both  interesting  and  important. 

Comparison  with  the  yield  of  the  Lode. If,   then,   the    COMSTOCK     LODE    is    SUppOScd 

to  have  derived  its  precious  metals  from  the  diabase,  we  should  expect  to 
find  that  it  yielded  dord  silver  containing  a  small  quantity  of  gold.  The 
gold  contents  has  actually  been  very  variable,  in  some  few  cases  exceed- 
ing the  value  of  the  silver  and  in  other  instances  amounting  to  only  a  fourth 
of  its  value.  The  Lode  has  bee'n  pretty  thoroughly  explored  to  a  depth  of 
2,500  feet,  and  the  extent  of  diabase  exposed  may  be  put  roughly  at  a 
length  of  8,000  feet  and  a  thickness  of  2,500  feet.  If  about  13  cents  per 
ton,  or,  say,  1  cent  per  cubic  foot,  has  been  extracted  from  this  mass,  the 


CHEMISTRY.  225 

total  amount  thus  accounted  for  is  $500,000,000.  Over  $300,000,000  have 
been  actually  put  upon  the  market,  and  nearly  $100,000,000  more  have 
probably  been  lost  in  tailings.  The  low-grade  quartz  not  extracted  most 
likely  contains  more  than  another  hvmdred  milHons,  but  the  sum  obtained 
by  calculation  is  nevertheless  a  fair  approximation  to  the  amount  which  the 
Lode  must  actually  have  contained.  On  the  other  hand,  if  an  attempt  be 
made  to  account  for  the  ore  on  any  other  supposition  than  that  it  was  derived 
from  the  diabase,  it  seems  very  difficult  to  give  a  plausible  explanation  for 
the  disappearance  of  the  gold  and  silver  which  appear  to  have  been  extracted 
from  this  rock. 

Other  rocks. — The  diorite  also  contains  precious  metals;  but 'while  dioritic 
vein  matter  is  highly  charged,  and  even  that  at  the  mouth  of  Bullion  ravine, 
which  is  very  solid  but  contains  some  pyrite  and  is  very  close  to  the  Lode, 
carries  a  notable  quantity,  that  from  the  head  of  the  same  ravine  shows  only 
a  trace  of  silver.  These  relations  are  the  reverse  of  those  observed  in  the 
diabase  and  appear  to  indicate  an  impregnation  from  the  Lode.  The  diorite 
also  contains  a  trace  of  gold.  More  could  hardly  have  been  e^cpected;  for, 
except  on  Cedar  Hill,  it  has  never  been  found  worth  while  to  treat  the  gold 
quartz  of  the  District,  and  the  Cedar  Hill  mines  have  yielded  but  little. 

The  andesites  and  the  quartz-poi'phyry  show  only  very  small  amounts 
of  silver,  but  the  metamorphic  diorite  contains  eight  cents  per  ton.  The 
analysis  also  shows  that  this  rock  is  highly  calcareous,  and  it  seems  not 
impossible  that  the  Justice  oi"e  body,  which  is  associated  with  the  meta- 
morphic diorite,  was  derived  from  it.  The  basalt,  on  the  contrary,  is 
nearly  as  rich  in  silver  as  the  older  diabase,  but  no  ore  is  likely  to  have 
been  extracted  from  it,  for  the  rock  is  not  only  the  freshest  in  the  Disteict, 
but  is  remarkably  fresh  for  any  region,  many  of  the  olivines  showing  no 
trace  of  attack. 

Lateral-secretion  theory  affirmed. — Ou  thc  wholc,  therefore,  the  chsmlcal  aud  geo- 
logical evidence  point  to  the  lateral-secretion  theory  as  the  true  explanation 
of  the  Washoe  ore  deposits,  and  to  the  augite  of  the  older  diabase  as  the 
source  of  the  important  ore  bodies.  It  is  worth  while  to  note  that,  accord- 
ing to  report,  many  of  the  famous  silver  mines  of  the  world  are  associated 
with  this  rock. 
15  0  L 


226  GEOLOGY  OF  THE  COMSTOOK  LODE. 

Nature  of  the  solvents. — As  has  been  sceii,  there  is  reason  to  suppose  that  the 
active  reagents  in  the  decomposition  of  the  minerals  of  the  diabase  were 
sulphhydi'ic  and  carbonic  acids.  These  acids  so  usually  reach  the  surface  in 
volcanic  regions  that  there  seems  no  necessity  for  examining  their  origin 
here,  but  it  may  be  pointed  out  that  solutions  of  sulphates  rising  through 
graphitic  slates,  such  as  form  in  part  the  foot  wall  of  the  Gold  Hill  mines, 
would  necessarily  be  reduced  to  sulphides.  Both  augite  and  plagioclase 
would  yield  to  the  attack  of  carbonic  and  hydrosulphuric  acids;  carbonates 
and  sulphides  of  the  alkalies  and  alkaline  earths  would  be  formed,  and  these 
are  solvents  for  quartz  and  sulphides  of  the  heavy  metals.  There  is  no 
difficulty,  therefore,  in  accounting  for  the  solution  of  the  materials  filling 
the  CoMSTOCK  Lode.  It  is  somewhat  less  easy  to  trace  the  precipitation  of 
the  ore  with  certainty.  Solutions  of  silica  in  water  containing  alkaline  car- 
bonates deposit  silicic  acid  only  on  evaporation,  not  on  cooling;  but  when 
sulphides  of  the  alkalies  are  also  present  a  reduction  of  temperature  is  fol- 
lowed by  the  precipitation  of  a  portion  of  the  silica.  Solutions  percolating 
from  the  east  country  into  the  main  fissure,  where  communication  with 
the  outer  air  was  less  impeded,  may  have  deposited  some  of  the  quartz  in 
consequence  of  cooling.  This  possibility,  however,  seems  scarcely  adequate 
to  explain  the  phenomena.  Vast  quantities  of  the  solvent  must  have  been 
necessary  to  carry  all  the  silica  occurring  on  the  Lode  ;  and  it  is  difficult  to 
understand  how  any  great  amount  of  cooling  can  have  taken  place.  If  hot 
solutions  are  supposed  to  have  issued  as  springs  along  the  croppings,  the 
influence  of  exterior  conditions  on  the  temperature  of  the  water  below  the 
surface  must  have  been  insignificant,  and  Sandberger  has  found  that  copious 
mineral  springs  deposit  sinter  about  their  orifices,  but  not  in  the  channels 
leading  to  them.  Even  if  the  solutions  may  be  supposed  not  to  have  over- 
flowed, being,  as  they  must  have  been,  in  communication  with  an  active 
source  of  heat,  they  would  have  been  maintained  at  a  nearly,  constant  tem- 
perature by  convection. 

Precipitation. — Silica  Is  vcry  readily  precipitated  from  solution,  and  it  is 
well  known  that  when  both  silica  and  carbonate  of  calcium  are  dissolved 
in  the  waters  of  hot  springs,  the  acid  is  deposited  near  the  source  and  calcite 
at  a  greater  distance.     Sandberger  states  that  when  such  solutions  become 


OHEMISTET.  227 

saturated  with  carbonates  the  siUca  is  precipitated.  If  so/  it  is  not  difficult 
to  understand  how  a  continuous  precipitation  of  silica  may  have  taken 
place  while  the  carbonates  were  carried  off  in  solution. 

It  has  been  explained  that  the  District  shows  very  small  evidences  of 
erosion  since  the  deposition  of  ore  began — less  than  one  would  suppose  com- 
patible with  the  deposition  of  quartz  from  flowing  springs  on  so  large  a 
scale.  The  District  presents  many  points  of  similarity  to  the  neighbor- 
hood of  Steamboat  Springs,  where  but  little  water  flows  ofi",  while  abundant 
columns  of  steam  constantly  rise  from  many  vents.  If,  as  seems  probable, 
the  condition  of  things  at  Washoe  was  similar,  the  precipitation  of  silica 
must  have  been  greatly  accelerated  by  concentration  of  the  solutions  through 
evaporation.  Precipitated  silica  is,  of  course,  in  great  part  amorphous,  but 
its  conversion  into  quartz  is  a  well-known  change. 

'  This  statement  is  no  doubt  founded  on  experiments,  of  which  I  have  failed  to  find  an  acconnt. 


CHAPTER    VII. 

HEAT  PHENOMENA  OF  THE  LODE. 

Sp:ction  1. 

GENERAL  DISCUSSION. 

High  temperatures  of  the  mines. — OiiG  of  the  pecuHarities  for  which  the  CoM- 
STOCK  Lode  has  been  famous  ever  since  deep  mining  began  upon  it,  is  the 
high  temperature  of  the  rock  and  of  the  water  encountered.  In  this  respect 
it  stands  alone  among  ore  deposits,  though  water  heated  to  125°  F.  has 
been  encountered  in  the  CHfford  mine  in  Wales,  and  very  hot  water  is  found 
in  the  superficial  workings  of  the  cinnabar  deposits  in  the  coast  range  of 
California.  On  the  3,000-foot  level  of  the  Comstock  floods  of  water  have 
entered  the  mines  at  170°  F.  Water  at  this  temperature  will  cook  food,  and 
will  destroy  the  human  epidermis.  Even  a  partial  immersion  in  it  is  there- 
fore fatal.  In  spite  of  very  rapid  ventilation,  the  air  in  the  underground 
galleries  is  often  intensely  heated  and  is  nearly  saturated  with  aqueous 
vapor.  Many  deaths  among  the  miners  have  occurred  from  prolonged 
exposure  to  these  unnatural  conditions,  which  also  add  immensely  to  the 
difficulties  of  geological  exploration. 

Normal  increment  of  heat. — A  great  many  luvestigations  have  been  made  during 
the  last  years,  in  many  parts  of  the  world,  on  the  increase  of  the  temperature 
from  the  surface  of  the  earth  downward.  The  observations  have  not  resulted 
in  establishing  a  uniform  rate  of  increase  in  any  locality,  nor  is  such  a  result 
to  be  expected  from  any  future  observations.  If  the  temperature  is  deter- 
mined in  a  freshly  drilled  hole  the  record  will  necessarily  be  too  high, 
because  the  surrounding  rock  is  heated  by  the  mechanical  action  of  the  drill. 
But  the  moment  the  rock  is  placed  in  communication  with  air  from  the 
surface,  or  with  water  from    higher  levels,  it  begins  to  cool  ofi".     Rocks  are 

228 


HEAT  PHENOMENA,  229 

always  more  or  less  fissured,  and  a  shaft  or  well  of  any  depth  commonly 
drains  the  surrounding  country,  so  that  water  from  a  higher  level  is  almost 
invariably  present  at  the  bottom.  If  a  shaft  is  kept  pumped  out,  the  equi- 
librium of  waters  at  a  lower  level  may  be  disturbed,  and  cixrrents  from 
greater  depths  will  then  rise  into  the  excavation.  Even  when  the  surface 
is  unbroken  it  is  well  known  that  there  are  usually  subterranean  cur- 
rents, the  course  of  which  is  determined  by  the  structure  of  the  rock, 
and  which  locally  interfere  with  the  regularity  of  the  isogeotherms.  While 
absolute  uniformity  in  the  increase  of  temperature  is  nowhere  to  be  expected, 
a  vast  number  of  observations  show  that  the  variations  are  usually  confined 
to  comparatively  thin  belts,  and  that  they  vibrate  about  a  rate  of  1°  F.  to 
from  50  to  60  feet  of  depth.  Sir  William  Thomson  makes  an  increase  of 
1°  F.  for  every  51  feet  of  descent  the  basis  of  his  calculations  on  the  secular 
cooling  of  the  earth.  The  marked  exceptions  occur  in  regions  where  there 
are  other  evidences  of  an  abnormal  temperature,  furnished  by  traces  of 
recent  volcanic  action  or  by  the  presence  of  hot  springs. 

Disturbing  effect  of  local  causes  in  mines. If     the     obsCrVatioUS     takcU     iu     VCrtical 

openings  of  small  diameter,  such  as  artesian  wells  and  mining  shafts,  are 
subject  to  fluctuations  from  local  causes  like  those  above  mentioned,  this 
must  be  to  a  much  greater  extent  the  case  in  an  extensive  and  complex 
system  of  mines,  such  as  those  which  are  being  worked  on  the  Comstock 
Lode.  The  country  is  honeycombed  to  a  depth  of  3,000  feet.  Above  150 
miles  of  galleries  have  been  driven,  besides  stopes  of  a  very  extensive 
character,  and  in  many  of  these  artificial  ventilation  has  been  going  on  for 
years.  On  account  of  the  great  heat,  the  ventilation  is  naturally  rapid,  and 
is  artificially  stimulated  to  the  greatest  possible  extent.  The  air  leaves  the 
mines  nearly  saturated  with  aqueous  vapor,  at  an  average  temperature, 
according  to  Mr.  Church,  of  92°  F.  In  this  way  an  enormous  quantity 
of  heat  has  been  abstracted  from  the  rock.  Although  before  the  opening 
of  the  mines  the  country  was  almost  absolutely  dry,  about  7,000,000  tons 
of  hot  water  are  now  yearly  pumped  from  the  Lode.  Mr.  Church  esti- 
mates that  the  heat  annually  abstracted  from  the  Lode  by  drainage  and 
ventilation,  without  considering  evaporation,  is  as  great  as  55,472  tons  of 
anthracite  produce  in  the  best  manufacturing  usage.      The  distui'bance  of 


230  GEOLOGY  OF  THE  COMSTOCK  LODE. 

the  natural  distribution  of  the  waters,  and  consequently  also  of  the  heat,  is 
further  indicated  by  the  immense  pressure  which  the  water  often  shows  on 
being  tapped  by  the  drills  in  the  lower  levels.  This  not  infrequently 
amounts  to  a  head  of  several  hundred  feet. 

Scattered  observations  cannot  agree  closely. — Taking  thesc  circumstauces  iuto  Consid- 
eration, it  appears  to  me  impossible  to  reach  any  accurate  result  by  discuss- 
ing in. detail  the  fluctuation  of  the  temperatures  observed  at  different  times 
in  different  portions  of  the  Lode.  Before  ground  was  broken  considerable 
variations  probably  existed  in  consequence  of  the  presence  of  convection 
currents.  Under  the  present  conditions  it  appears  from  the  foregoing  that 
great  fluctuations  from  a  regular  law  of  increase,  and  great  anomalies  which 
cannot  be  immediately  traced  to  their  sources,  must  inevitably  occur. 

A  first   approximation  from  such  data. BarOU     V.     Richtliofcn,   althoUgh    insistiug 

strongly  on  the  abundant  evidences  of  solfatarism,  mentions  no  abnormal 
temperatures.  Mr.  King  gives  a  table  of  observations,  from  which  it  ap- 
pears that  the  average  temperature  of  the  mine  waters,  from  the  surface  to 
the  700-foot  level,  is  between  70°  and  75°  F.  At  a  depth  of  about  1,100 
feet  he  found  water  at  108°  F.  Mr.  King  remarks  :  "That  to  the  waters  is 
due  the  temperature  of  the  whole  interior  of  the  Lode  is  evident  from  the 
fact  that  they  average  a  few  degi'ees  higher  than  the  clays  or  rocky  mate- 
rial." He  notes  only  one  instance  in  which  the  rock  and  water  showed  the 
same  temperature.  Mr.  Church  made  many  careful  observations,  which  he 
has  very  fully  discussed.  He  estimates  the  mean  temperature  of  freshly 
exposed  surfaces  on  the  2,000-foot  level  at  130°.  The  water  with  which 
the  Gold  Hill  mines  were  flooded  in  the  winter  of  1880-'81  entered  on  the 
3,000-foot  level.  It  was  repeatedly  tested  by  the  officers  of  the  mines,  and 
by  myself,  and  was  found  to  have  a  temperature  of  170°  F.  This  water 
was  first  struck  at  a  depth  of  3,080  feet,  by  a  drill  hole  from  the  bottom  of 
the  Yellow  Jacket  shaft.  Taking  into  consideration  that  170°  is  not  an 
average,  but  probably  a  maximum  for  this  depth,  these  data  indicate 
roughly  a  nearly  uniform  increase  of  temperature  of  about  1°  for  every  28 
feet.^  If  the  attempt  be  made  to  discuss  the  observations  in  detail,  great 
irregularities  will  be  found.     As  Mr.  Church  very  pertinently  remarks,  "  the 

'  More  exactly  an  increase  of  1°  in  28.7  feet  for  the  interval  of  1,650  feet  between  the  350-foot  and 
the  2,000-foot  levels,  and  of  1"  in  27J  feet  for  the  1,100  feet  between  the  2,000  and  3,100-foot  levels. 


HEAT  PHENOMENA.  231 

mining  works  do  not  follow  the  lines  of  heat  manifestation,  but  intersect 
them  in  every  possible  manner." 

Better  data  lately  obtained. — Thanks  to  Mr.  Churcli,  better  data  have  been  ob- 
tained since  his  memoir  was  written.  At  his  suggestion  frequent  observa- 
tions have  been  made  on  the  temperature  of  the  rock  and  the  water 
encountered  in  sinking  the  Combination,  the  Yelloiv  Jacket,  and  the  Forman 
shafts.  A  long  series  of  observations  has  also  been  made  in  the  Sutro  Tun- 
nel These  observations  and  their  discussion  will  be  found  in  the  second 
section  of  this  chapter.  Though  they  might  properly  be  introduced  here 
their  voluminous  character  makes  it  more  expedient  to  consider  them  sepa- 
rately. The  two  chains  of  reasoning  may  be  regarded  as  parallel  argu- 
ments on  the  same  subject. 

Explanations  of  the  heat, — Various  cxplanatlons  have  been  offered  to  account 
for  the  prevalence  of  high  temperatures  on  the.CoMSTOCK.  The  source  of 
heat  has  been  sought  in  friction,  in  the  oxidation  of  pyrite,  in  the  kaoliniza- 
tion  of  feldspar,  and  in  volcanic  action. 

That  heat  must  have  resulted  from  the  faulting  action  there  can  be  no 
doubt,  but  the  whole  tendency  of  the  evidence  is  so  strongly  against  the 
application  of  Mr.  Mallet's  hypothesis  of  terrestrial  heat  to  this  instance, 
that  a  discussion  seems  unnecessary.  The  oxidation  of  pyrite,  too,  is  a  very 
subordinate  phenomenon  on  the  Comstock.  It  is  well  known  that  vari- 
ous occurrences  of  pyrite  differ  greatly  in  their  behavior  toward  oxidizing 
agents.  That  found  on  the  Comstock  is  for  the  most  part  very  stable,  and 
often  remains  exposed  for  years  with  no  greater  effect  than  tarnishing. 
Most  of  the  water  from  the  Lode,  too,  shows  but  a  small  amount  of  sul- 
phates. Indeed,  there  is  much  more  reason  to  suppose  that  the  formation 
of  pyrite  is  still  in  progress,  on  a  small  scale,  than  that  the  decomposition 
of  this  mineral  is  the  source  of  heat. 

statement  of  the  kaoiinization  hypothesis. — The  hypothesis  that  the  high  tcmpera- 
ture  is  due  to  the  kaoiinization  of  feldspar,  appears  to  rest  on  two  positive 
grounds,  viz.,  that  flooded  drifts  have  been  observed  to  grow  hotter,  and  that 
the  solidification  of  water  liberates  heat.  In  the  argument  supporting  this 
hypothesis,  its  author  makes  the  following  statement : 

"  The  direct  evidence  that  heat  is  produced  when  water  is  brought  in 


232  GEOLOGY  OF  THE  COMSTOOK  LODE. 

contact  with  these  rocks  is  of  constant  occurrence  in  the  mines,  and  is 
offered,  in  fact,  whenever  a  pump  breaks  or  is  stopped  for  any  reason,  and 
water  rises  upon  a  partially  decomposed  seam.  A  case  of  this  kind  in  the 
Caledonia  is  of  more  than  ordinary  interest,  for  the  reason  that  this  was  a 
cool  mine,  both  rock  and  water  being  but  little  above  ordinary  tempera- 
tures. The  heat  of  the  air  in  the  drift  was  probably  not  above  90°  F.,  but 
after  lying  twenty-four  hours  under  water  a  very  marked  change  took  place. 
The  water  had  reached  a  thick  seam  of  the  kind  that  is  solid  enough  when 
dry,  but  swells  with  great  force  when  wet.  The  1 2-inch  timbers  were  all 
splintered,  and  the  temperature  of  the  level  had  risen  probably  to  110°, 
though  no  observation  was  taken.  Still  the  fact  of  increased  temperature 
and  of  increase  from  this  cause  alone  was  undoubted.  Since  that  time  the 
Julia,  Savage,  and  Hale  <&  Norcross  mines  have  all  been  flooded  and  subse- 
-quently  drained.  The  Norcross  has  a  fine  current  of  fresh  air,  and  I  have 
not  observed  any  complaint  of  its  condition,  but  both  the  other  mines  were 
reported  to  be  extremely  hot  after  their  submersion.  They  were  very  much 
above  their  usual  temperature,  and  work  was  frequently  stopped  to  allow 
them  to  cool  down.  Such  evidences  cannot  take  the  place  of  exact  labora- 
tory experiments,  but  they  are  just  as  incontestable  proof  of  the  fact  of 
heat,  and  high  heat,  from  kaolinization,  as  if  we  had  its  precise  measure." 

Criticism  on  the  evidence. — It  Will  be  obscrvcd  that  it  is  Hot  Stated  when  the 
flooding  of  the  drift  in  the  Caledonia  occurred,  or  who  estimated  the  temper- 
atures; nor  yet  whether  the  water  of  this  particular  flood  was  warm  or  cold. 
With  regard  to  the  flooding  of  the  Julia,  Savage,  and  Hale  &  Norcross 
mines,  the  only  event  of  the  kind  known  to  me  was  that  which  occurred  in 
1876.  This  flood  lasted  for  three  years.  Soon  after  its  commencement  an 
official  report  of  the  superintendent  gave  the  temperature  of  the  water  at 
139°,  and  Mr.  Church  reports  it  later  (apparently  early  in  1878)  as  154°.' 
The  great  heat  of  these  mines  appears  to  require  no  further  explanation. 
I  am  not  able"  to  confirm  the  observation  that  flooded  drifts  grow  hotter, 
except  when  the  water  of  the  flood  enters  the  workings  at  a  high  tempera- 

'  This  change  of  temperature  is  not  remarkable  and  has  not  been  advanced  in  favor  of  the  chem-" 
ical  theory  of  the  heat,  for  many  millions  of  gallons  were  pumped  from  the  flooded  mines.  Streams  per- 
colating from  a  large  body  of  heated  -water  through  new  channels  in  comparatively  cool  rock  will  at 
first  be  cooled ;  but  they  will  grow  warmer  as  the  rock  ia  gradually  raised  to  the  temperature  of  the 
water  at  the  source. 


HEAT  PHENOMENA.  233 

ture.  There  are  many  miles  of  drifts  on  the  Comstock  flooded  to  a  greater 
or  less  extent;  but  a  great  number  of  observations  made  by  my  party  show 
that  the  water  is  hottest  when  it  issues  from  the  rock,  and  cools  off  by 
standing  in  the  workings.  When  the  water  at  its  entrance  is  tepid  or  cool, 
it  appears  to  remain  so  indefinitely,  even  though  it  may  be  stagnant 

Examination  of  the  theory  of  kaoiinization. — While  uo  fact  Can  be  better  established 
than  that  the  solidification  of  water  liberates  heat,  no  direct  conclusions  can 
be  drawn  from  it  as  to  the  relations  of  the  complex  process  of  kaoiinization. 
The  constitution  of  the  unisilicates  is  still  very  obscure,  and  there  is  no 
unanimity  of  opinion  among  mineralogical  chemists,  even  as  to  the  formu- 
las by  which  they  should  be  represented,  while  almost  nothing  is  known  of 
the  reactions  which  go  on  during  decomposition.  It  may  not  be  amiss, 
however,  to  examine  the  question  from  a  theoretical  point  of  view. 

Feldspar  assumed  as  representative. — As  has  been  showu  in  Chapter  III.,  the  feld- 
spars  of  the  diorite  and  diabase  which  form  the  walls  of  the  Comstock  are 
apparently  labradorite  and  oligoclase  Whether  Professor  Tschermak's 
theory  of  the  feldspar  group  is  correct  or  not,  a  mixture  of  these  feldspars 
may  for  the  present  purpose  be  regarded  as  a  compound  of  one  molecule  of 
anorthite  and  one  of  albite.  The  mixed  or  intermediate  (andesine)  feldspar 
may  then  be  written 

Na^Al  )  ^^^  ) 

lAn  +  lAb  =  (CaAl)Si'0«-|-^.,  Si^Oi^^Na^AlSSi^O^*. 

^^"       ^  SiM 

First  step  of  decomposition. — The  examination  of  thin  sections  leads  me  tb  be- 
lieve that  the  first  change  in  the  feldspars  of  the  Washoe  rocks  is  the 
formation  of  calcdte,  accompanied  by  a  separation  of  silica.  The  formation 
of  sodium  silicate  probably  takes  place  at  the  same  time,  but  is  not  traceable 
by  optical  means,  for  it  will  dissolve,  and  either  pass  out  of  the  rock  or 
become  diffused  through  it.     If  from  the  above  formula 

CaO  +  SiO^  and  Na'^SiO' 
are  subtracted,  it  becomes 

Al 

Al  >  Si*0^^ 

Si' 


234  GEOLOGY  OF  THE  OOMSTOCK  LODE. 

The  feldspars  of  the  massive  and  metamorphic  rocks  are  ordinarily 
fresh,^  and  they  appear  to  decompose  only  under  peculiar  conditions,  the 
details  of  which  are  not  fully  understood,  but  the  circumstances  point  to 
the  intervention  of  external  energy.  Such  behavior  is  characteristic  of 
compounds  the  formation  of  which  is  accompanied  by  a  liberation  of  heat. 
The  silicates  containing  a  single  base  appear  to  liberate  but  a  very  small 
amount  of  heat,  for  the  thermal  effect  even  of  the  formation  of  sodium 
sihcate  is  very  small  indeed,  and  that  of  calcium  and  aluminium  siKcates 
is,  by  inference,  smaller  still.  The  separation  of  the  feldspars  into  sihcates 
of  the  earths  will  probably,  therefore,  be  accompanied  by  the  absorption 
of  heat,  and  so  will  the  solution  of  sodium  silicate.  I  know  of  no  experi- 
ments to  show  precisely  what  is  the  thermal  effect  of  the  conversion  of 
calcium  silicate  into  calcium  carbonate,  but  the  behavior  of  the  carbonate 
and  silicate  of  sodium,  and  of  calcic  carbonate,  leaves  little  doubt  that  it 
must  be  the  evolution  of  a  small  amount  of  heat,  less  than  that  evolved 
by  the  formation  of  calcium  carbonate. 

Formation  of  kaolin. — If  kaoliu  rcsults  from  the  decomposition  of  these  feld- 
spars, there  must  be  a  still  further  separation  of  silica,  and  an  introduction 
of  hydrogen.  The  structural  formula  adopted  suggests  interesting  possi- 
bilities. It  is,  namely,  by  no  means  impossible  that  the  siHcon  represented 
in  the  last  formula  as  basic  should  be  replaced  by  hydrogen  by  the  reaction 

2Si  +  4H20  =  2Si02-|-8H. 
Were  this  the  case,  the  result  would  be  silicic  anhydride  and 

=41>Si*0i«, 

w) 

or  twice 

A120^2SiO^-^2H^O 

(the  ordinary  formula  for  kaohn),  if  the  water  is  regarded  as  combined. 
The  heat  hberated  by  the  reaction 

2Si-f4H^Oz=2SiO^-f  8H 

1  It  has  been  already  remarked  that  the  decompositiou  of  .au  iusignificant  percentage  of  afeldspar 
crystal  rohs  it  of  its  transparency.  Many  dull,  chalky-looking  feldspars,  when  seen  nnder  the  micro- 
scope, prove  to  he  very  slightly  altered. 


HEAT  PHENOMENA,  235 

can  be  calculated;  for  the  combination 

2H  +  0  =  H^O  (solid)  liberates    70,400  units  of  heat, 
Si  +  20  =  SiO'  liberates  211,100  units  of  heat, 

and  therefore 

.    2Si  +  4H^0  =  2SiO-  +  8H  liberates  140,600  units. 

The  molecular  weight  of  the  andesine  feldspar  under  discussion  is  757.6, 
and  the  heat  liberated  per  unit  of  weight  would  be 

140,600     1  o^      V 
,^^  =  186umts. 

The  specific  heat  of  feldspars  varies  from  0.183  to  0.196.  If  the  ande- 
sine under  discussion  is  supposed  to  have  a  specific  heat  of  0.1 86,  the  tem- 
perature resulting  from  the  substitution  of  hydrogen  for  silicon  would  be 
1000°  C. 

The  aH^o  is  water  of  hydration. — If,  then,  the  watcr  ill  kaoliu  were  chemically 
combined,  a  temperature  would  be  produced  much  above  that  known  to  be 
sufficient  to  expel  the  water  from  clay,  and  the  only  inference  I  can  draw  is 
that  the  water  is  not,  as  has  sometimes  been  maintained,  chemically  combined, 
but  is  merely  water  of  hydration.  The  latter  view  (which  is  also  generally 
held)  is  further  supported  by  the  varying  amounts  of  water  which  various 
analysts  have  found  in  kaolin.  As  is  well  known,  Tschermak  even  denies 
that  kaolin  is  a  product  of  the  decomposition  of  plagioclase,  affirming  that 
the  resulting  hydrated  aluminium  silicate  contains  but  a  single  molecule  of 
water. 

Nothing  known  of  the  heat  of  hydration  of  kaolin. Thc  pUrpOSC  of  the  forCgoing  argu- 
ment is  to  show  that  if  any  considerable  quantity  of  heat  is  evolved  during 
kaolinization,  it  must  in  all  probability  be  due  to  the  simple  hydration  of 
aluminium  silicate.  But  of  the  heat  liberated  by  the  hydration  of  salts  little 
is  known,  except  (1)  that  the  quantity  is  usually  small,  (2)  that  it  is  some- 
times negative,  and  (3)  that  the  different  molecules  of  water  combine  with 
differing  amounts  of  energy,  indicating  that  the  nature  of  their  union  difiers. 
Of  the  heat  of  hydration  of  kaolin  we  know  nothing  specific,  nor  am  I 


236  GEOLOGY  OF  THE  COMSTOCK  LODE. 

aware  of  any  analogy  wHich  indicates  a  likelihood  that  it  is  sufficient  to 
account  for  the  heat  phenomena  of  the  Comstock  Lode. 

Experiments  on  kaoiinization. — lu  the  hope  of  reaching  more  satisfactory  results 
regarding  kaolinization  than  observation  or  theoretical  considerations  yielded, 
I  requested  Dr.  Barns  to  undertake  experiments  with  a  view  to  testing  the 
asserted  rise  of  temperature  when  the  Washoe  rocks  are  brought  in  contact 
with  water. 

Material  selected. — The  rock  Selected  was  from  a  mass  cut  by  the  Sutro  Tun- 
nel in  the  Savage  claim,  just  before  the  tunnel  strikes  the  vein.  It  is  a  dia- 
base, and  the  freshest  encountered  under  ground  opposite  that  portion  of 
the  Lode  which  has  been  considerably  productive.  It  is  described  under 
slide  1 8,  and  its  analysis  is  given  at  the  end  of  Chapter  III.  Lest  it  should  be 
objected  that  this  rock  had  escaped  decomposition  through  an  exceptional 
striicture  or  a  local  variation  in  chemical  composition,  it  may  be  remarked 
that  no  trace  of  such  a  difference  is  perceptible  either  macroscopically  or 
microscopically,  while  its  exemption  from  decomposition  is  fully  accounted 
for  by  the  character  of  its  occurrence.  This  mass,  like  most  of  the  fresher 
rocks  in  the  District,  is  protected  by  clay  seams  which  have  prevented  the 
access  of  aqueous  currents.  The  hanging  wall  of  the  Comstock  is  to  so  large 
an  extent  obliterated  by  decomposition  that  many  observant  miners  deny  its 
existence,  but  at  this  particular  spot  no  vein-wall  could  be  better  defined. 
It  is  marked  by  a  compact  smooth  clay  a  foot  or  more  in  thickness,  imme- 
diately over  which  lies  the  mass  of  rock  referred  to.  This  is  further  pro- 
tected, though  not  so  clearly,  by  other  clay-seams  to  the  east,  and  is  much 
less  shattered  than  the  rock  elsewhere. 

Method  adopted. — The  rock  was  reduced  to  a  gravel  and  placed  within  a 
well-packed  steam  jacket.  Steam  was  supplied  from  a  boiler  beneath,  in 
which  the  water  was  kept  at  a  constant  level  and  constantly  boihng.  The 
difference  of  temperature  between  the  rock  and  the  inclosing  steam  was 
measured  by  a  thermopile.  The  electro-motive  force  was  so  compensated 
that  a  variation  of  0.001°  C.  was  clearly  indicated,  and  the  experiments 
extended  over  five  weeks  with  only  four  interruptions.  The  whole  plan  of 
the  investigation  was  worked  out  by  Dr.  Barus,  who  wiU  describe  it  in 
detail  in  a  separate  chapter.     The  appliances  at  his  command  were  few  and 


HEAT  PHENOMENA.  2^7 

simple,  but  they  were  employed  with  such  ingenuity  as  to  enable  him  to 
obtain  very  accurate  results.  The  execution  of  the  experiments  was  most 
conscientious  and  laborious. 

No  positive  results  obtained. — The  tcmpcrature  of  the  rock-mass  never  rose 
above  that  of  the  surrounding  steam.  The  rock  seemed  wholly  unaffected 
by  the  process,  except  that  the  fragments  were  more  or  less  coated  with 
a  fine  dust,  probably  due  to  the  salts  contained  in  the  water,  which  was 
obtained  from  the  Virginia  Water  Company's  pipes. 

Little  kaoiinization  at  Washoe. — Some  time  after  tho  oxecution  of  these  experi- 
ments a  special  examination  of  the  slides  and  a  comparison  of  chemical 
analyses  led  me  to  the  conclusion  that  there  has  been  only  a  trifling  amount 
of  kaolinization  in  the  Washoe  rocks.  This  fact  makes  the  experiments 
none  the  less  important,  for  the  heat  of  the  Lode  might  be  due  to  other 
chemical  changes  than  kaolinization. 

Conclusions  regarding  the  hypothesis. lu  short,   the    obserVatloUS  aS    tO  thc   risC  of 

temperature  of  flooded  drifts  lack  confirmation;  experiment  fails  to  show 
that  hot  water  or  steam  have  any  action  on  the  east  country  rock  of  the  Lode; 
there  appear  no  theoretical  grounds  for  the  assertion  that  kaolinization 
would  produce  a  considerable  amount  of  heat,  and  no  evidence  that  any 
considerable  amount  of  kaolinization  has  gone  on  in  the  District.  It  is  still 
possible  that  when  kaolinization  occurs  heat  is  liberated.  It  is  also  possible 
that  at  temperatures  above  212°  and  at  pressures  above  one  atmosphere, 
feldspars  are  kaolinized  near  the  Comstock  fissure,  but  it  no  longer  appears 
reasonable  to  ascribe  the  heating  of  drifts,  which  are  nearly  at  the  normal 
pressure,  to  the  action  of  water  below  the  boiling  point  upon  the  rock. 
The  scene  of  kaolinization,  if  it  exists  at  all,  must  therefore  be  at  great 
depths,  such  as  are  indicated  in  the  discussion  of  the  increase  of  tempera- 
ture from  the  surface  downward.  It  cannot  be  demonstrated  that  the  heat 
of  the  Comstock  is  not  due  to  the  prevalence  at  unknown  depths  and  press- 
ures of  a  chemical  change  of  unknown  thermal  relations,  neither  is  there 
any  evidence  that  it  does  arise  from  such  a  cause;  and  the  suggestion  that 
the  heat  of  the  Steamboat  Springs  and  the  ordinary  variations  of  earth  tem- 
peratures are  induced  by  kaolinization,  is  therefore  foreign  to  the  subject  of 
this  memoir. 


238  GEOLOGY  OF  THE  OOMSTOCK  LODE. 

soifataric  action. — The  Only  remaining  supposition  is  that  which  connects 
the  heat  of  the  Comstock  with  the  chain  of  volcanic  phenomena.  What 
is  known  as  soifataric  action  is  ill  understood,  and  must  remain  so  until 
many  of  the  mysteries  of  vulcanism  have  been  made  plain;  but  of  certain 
facts  there  is  no  doubt.  In  the  neighborhood  of  active  volcanoes,  and  often 
also  in  regions  where  eruptions  have  ceased,  gases  and  water  charged  with 
more  or  less  active  reagents  reach  the  surface  through  crevices.  In  its 
earliest  stages  a  soifataric  spring  frequently  emits  gas  or  water  charged 
with  fluorine  and  chlorine  compounds,  which  are  replaced  at  a  later  stage 
by  hydrosulphuric  and  carbonic  acids.  The  action  of  these  reagents  on  the 
rocks  is  manifold,  but  usually  gives  rise  to  characteristic  appearances,  such 
as  bleaching,  accompanied  by  an  extraction  of  a  smaller  or  greater  por- 
tion of  the  bases.  The  appearances  due  to  solfatarism  are,  of  course, 
accurately  known,  from  immediate  observation  in  the  neighborhood  of  active 
volcanoes.  On  the  other  hand,  it  is  very  seldom  that  effects  likely  to  be 
confounded  with  those  of  solfatarism  are  found  at  any  great  distance  from 
localities  marked  by  the  occurrence,  present  or  past,  of  volcanic  eruptions. 
No  two  phenomena  in  geology  are  more  intimately  connected  than  vol- 
canoes and  solfataras.  The  connection  between  ore  deposits  and  eruptive 
rocks  is  also  in  a  large  proportion  of  cases  a  very  close  one,  and  where 
ore  deposits  and  evidences  of  soifataric  action  are  found  together  in  a  vol- 
canic region,  it  is  certainly  natural  to  conclude  that  an  abnormal  temperature 
of  the  rock  and  water  is  also  due  to  vulcanism.  The  burden  of  proof 
rests  on  him  who  offers  any  other  explanation. 

Decomposed  area  at  Washoe. — Extremo  alteration  is  for  the  most  part  limited  to 
the  area  lying  between  the  Comstock  and  the  Occidental  Lodes,  though  it 
also  extends  up  some  of  the  ravines  to  the  west  of  the  great  vein.^  Even 
within  this  area  there  are  great  variations  in  the  degree  of  decomposition. 
While  a  portion  of  the  rock  on  the  surface  is  tolerably  well  preserved,  there 
are  belts  nearly  parallel  to  the  Lode,  in  which  it  is  so  altered  that  it  might 
be  mistaken  for  more  or  less  discolored  chalk.  These  belts  can  be  followed 
under  ground,  and  retain  in  dip  as  in  strike  an  approximate  parallelism  to 
the  vein.  Towards  the  edges  of  the  surface  area  it  is  common  to  find 
nodules  of  rock  in  place  which  are  fairly  fresh  at  the  center,  but  show  pro- 

— • "  ■ 

1  See  Fig.  Ij  page  73. 


HEAT  PHENOMENA.  239 

gressive  decomposition  towards  the  outside.  Large  masses  of  fresh  rock 
also  occur  in  a  similar  way,  as  has  been  described  in  the  discussion  of  pro- 
pylite.  It  is  clear  from  these  occurrences  that  had  the  decomposing  action 
been  prolonged  sufficiently,  no  undecomposed  rock  would  .have  remained. 
Under  ground  the  decomposition  is  more  universal,  if  one  may  judge  from 
the  Sutro  Tunnel.  From  Shaft  II.  to  the  Lode  no  fresh  rock  is  exposed  by 
the  tunnel,  except  the  small  mass  of  diabase  close  to  the  hanging  wall  which 
has  been  referred  to.  This  marked  difference  between  the  superficial  and 
subterranean  rocks  should  be  considered  in  connection  with  one  of  the 
deductions  made  in  discussing  the  structural  results  of  faulting — viz.,  that 
the  country  has  undergone  but  little  erosion  since  the  deposition  of  the  ore. 
Indeed,  it  may  be  regarded  as  independent  evidence  tending  to  the  same 
conclusion. 

Rocks  involved. — The  three  rocks  which  occur  in  the  belt  of  highly  decom- 
posed east  country  are  diabase,  hornblende-andesite,  and  augite-andesite. 
The  andesites  are  found  extensively  in  other  portions  of  the  District, 
where,  however,  they  are  decomposed  to  but  a  trifling  extent.  There  is 
no  reason  known  to  me  to  suppose  that  the  decomposed  andesites  are  of 
different  eruptions  from  the  fresh  occurrences ;  on  the  contrary,  the  decom- 
position dies  out  gradually  in  continuous  areas.  Neither  is  there  any 
evidence  that  the  fresh  and  the  altered  masses  are  of  a  different  composition. 

Evidence  of  an  external  cause. — Had  thc  resolutloH  of  the  complcx  rock  min- 
erals into  simple  compounds  been  spontaneous,  the  nodules  of  rock  described 
could  not  have  foi'med,  for  the  action  must  have  been  nearly  uniform 
throughout.  Neither  could  they  have  been  formed  if  the  presence  of  moist- 
ure had  been  sufficient  to  induce  decomposition,  for  all  rocks,  excej)t  perhaps 
obsidian,  are  permeable  by  water.  Solutions  of  carbonic  acid,  hydrosul- 
phuric  acid  or  the  like,  on  the  other  hand,  if  brought  in  contact  with  com- 
pact masses  of  material  susceptible  to  their  action,  would  grow  weaker  as 
they  penetrated  towards  the  centers  of  blocks,  and  would  bring  about  just 
such  results  as  those  referred  to. 

Evidence  that  the  solutions  ^scended. — If  surface  watcrs  had  producod  tlic  decompo- 
sition, the  andesites  at  the  surface  throughout  the  District  would  have  suf- 
fered nearly  uniformly,  and  the  amount  of  decomposition  must  have  decreased 


240  GEOLOGY  OF  THE  COMSTOOK  LODE. 

as  greater  depths  were  readied.  If,  subsequent  to  the  decomposition,  erosion 
had  taken  place,  the  rocks  at  lower  elevations  would  be  found  fresher  than 
those  on  the  hills.  The  reverse  is  the  case.  But  if  decomposition  was  pro- 
duced by  waters  rising  from  great  depths,  the  area  of  alteration  would 
dejjend  on  the  structure  of  the  rock,  on  the  existence  of  fissures  through 
which  they  could  reach  the  surface,  and  from  which  the}^  could  act  upon 
the  material  bounded  by  these  fissures;  which  accords  with  the  observations. 
Moreover,  the  resemblance  of  the  products  of  decomposition  in  this  Dis- 
trict to  those  occurring  in  solfataric  regions  is  very  strong,  and  their 
dissimilarity  to  those  produced  by  ordinary  surface  action  equally  great. 

These  considerations  appear  to  me  conclusive  that  the  decomposition 
was  efi'ected  by  aqueous  currents  rising  from  lower  depths,  and  that  these 
currents  carried  in  solution  reagents  capable  of  producing  the  effects  familiar 
in  solfataras. 

Nature  of  the  reagents. — There  is  somc  positive  evidence  as  to  what  these 
reagents  were,  for  the  water  struck  in  the  Yellow  Jacket  at  3,080  feet  from 
the  surface  was  so  strongly  charged  with  hydrogen  sulphide  as  seriously  to 
inconvenience  the  miners,  and  evidence  is  given  in  the  chapter  on  chemistry 
that  hydrosulphuric  acid  must  have  played  an  important  part  in  the  rock 
decomposition.  The  Steamboat  Springs,  which  lie  on  a  fissure  parallel  to  the 
CoMSTOCK,  and  on  the  opposite  side  of  the  Virginia  range,  are  also  charged 
with  solfataric  gases. 

Origin  of  the  reagents  volcanic. — There  is  uo  conccivable  reaction  between  water 
and  the  components  of  the  eruptive  rocks,  which  would  have  produced 
hydrogen  sulphide,  and  the  other  solfataric  gases.  Their  origin  must,  there- 
fore, be  sought  outside  of  and  below  these  eruptive  rocks.  It  would  cer- 
tainly be  permissible  to  argue  immediately  from  the  agency  of  solfataric 
gases  to  volcanic  action,  but  it  may  also  be  suggested  that  the  vast  quantity 
of  hydrosulphuric  and  carbonic  acids  which  have  been  consumed  could  not 
have  been  produced  at  low  temperatures,  and  that,  when  formed  at  unknown 
but  certainly  great  depths,  they  could  have  been  brought  to  the  surface  or 
the  mines  only  by  convection  currents,  which  were  stimulated  by  heat. 
These  considerations  force  me  to  the  belief  that  below  the  Comstock,  per- 
haps at  a  depth  of  three  or  more  miles,  there  is  a  large  body  of  highly 


HEAT  PHENOMENA.  241 

heated  rock  in  contact  with  sedimentary  material.  The  well-known  reac- 
tions which  take  place  under  such  circumstances  in  the  presence  of  water 
have  produced  soHataric  gases  as  long  as  the  supply  of  sulphates  and  of 
reducing  agents  held  out.  Of  these  there  is  now  a  mere  trace.  Whether 
this  highly  heated  rock  is  part  and  parcel  of  the  surface  rocks  of  the 
Washoe  District  is  a  question  which  can  only  be  answered  in  terms  of 
probabilities ;  yet  as  these  rocks  must  have  come  from  a  focus  of  volcanic 
action  in  about  the  same  vertical  line,  the  chances  are  certainly  in  favor  of 
the  supposition  that  the  high  temperature  of  the  Lode  is  a  later  member  of 
the  series  of  phenomena,  of  which  the  ejection  of  the  younger  hornblende- 
andesite,  or  possibly  of  the  basalt,  was  an  early  manifestation. 

The  rocks  all  moist. — The  disscmiuation  of  heat  through  the  rocks  of  the 
CoMSTocK  has  been  regarded  by  one  geologist  as  a  point  very  difficult  of 
explanation.  He  regarded  the  rocks  as  dry,  and  assuming  their  conductivity 
to  be  the  same  as  that  of  the  Calton  Hill  trap,  which  Sir  William  Thomson 
has  made  famous,  he  found  the  transmission  of  heat  insufficient  to  account 
for  the  facts.  The  rocks  are  in  great  part  dry,  as  miners  use  the  word — i.  e., 
many  exposures  do  not  drip  water;  but  though  paying  especial  attention  to 
the  subject,  I  found  none  which  were  not  moist.  Chips  and  specimens,  for 
example,  always  changed  color  after  half  an  hour's  exposure  to  dry  air, 
except  when  taken  from  flakes  which  were  already  partially  separated  from 
the  mass  and  exposed  to  a  drying  current.  The  rocks  of  the  District  are 
not  glassy  but  crystalline,  and  that  such  rocks  in  the  immediate  neighbor- 
hood of  vast  bodies  of  water  at  pressures  equivalent  to  a  head  of,  say,  from 
1,000  to  3,000  feet,  ever  since  they  cooled  many  thousand  years  ago, 
should  remain  dry,  would  be  strange  indeed,  and  quite  opposed  to  all  that 
is  known  of  the  permeability  of  rocks  by  water.  But  when  it  is  taken  into 
consideration  that  far  more  than  99  per  cent,  of  this  rock  is  highly  decom- 
posed, it  is  almost  inconceivable. 

Source  of  the  water  unexplained. — Tho  source  of  tho  Water  convcyiug  the  heat  to 
the  CoMSTOCK  is  somewhat  mysterious.  The  country  is  a  sage-brush  desert, 
and  the  rainfall  is  not  over  ten  inches.  The  slopes  are  steep  and  the  evapora- 
tion immense.  The  mines  are  now  so  deep  that  they  might  di-ain  a  large 
extent  of  country,  but  great  quantities  of  water  were  met  with  when  the 
workings  were  within  a  few  hundred  feet  of  the  surface  and  could  appar- 

16   0   L 


242  GEOLOGY  OF  THE  COMSTOGK  LODE. 

ently  drain  but  a  very  small  area.  Before  mining  began,  however,  little 
or  no  water  issued  from  the  surface.  When  the  first  floods  were  encoun- 
tered it  was  supposed  that  there  must  be  great  accumulations  of  water  in 
subterranean  caves,  and  that  water-ways  leading  to  them  had  been  cut 
by  the  workings.  But  no  such  openings  were  ever  reached  in  the  mines, 
and  it  came  to  be  supposed  that  the  water  had  accumulated  in  the  inter- 
stices of  shattered  rock  masses.  Broken  as  the  rock  is,  however,  it  is  very 
closely  packed,  so  that  the  interstitial  space  is  but  small,  and  considering 
the  vast  quantities  of  water  which  have  been  pumped  from  the  mines,  I 
cannot  think  the  explanation  adequate.^  The  pressure  under  which  the 
water  is  frequently  met  is  a  significant  feature  of  its  occurrence.  Though 
there  may  be  other  workings  on  the  same  level,  and  though  the  country 
above  may  be  extensively  opened  up,  a  new  source  will  sometimes  show  a 
head  of  several  hundred  feet.  The  deeper  the  point  at  which  the  water  is 
struck  the  hotter  it  usually  is,  and  there  appears  to  be  some  tendency  of  the 
temperature  of  the  water  from  a  single  source  to  increase  as  it  is  drained. 
But  if  it  were  accumulated  in  a  mass  of  shattered  rock  of  limited  extent,  the 
water  and  the  rock  throughout  the  entire  space  would  necessarily  assume  a 
perfectly  uniform  temperature,  and  channels  tapping  such  an  accumulation 
at  different  levels  would  emit  streams  of  the  same  temperature.  As  has 
been  seen,  the  rock  is  commonly  cooler  than  the  water,  and  the  general 
reasoning  in  the  foregoing  paragraphs  points  to  the  rise  of  currents  from 
great  depths.     An  attempt  will  be  made  to  reconcile  these  facts. 

Hypothesis  of  its  origin  in  the  Sierra. lu  thc   Gold  Hill  miuCS  tllC  foOt  Wall  of  thc 

Lode  in  the  lower  levels  is  composed  of  metamorphic  rocks  dipping  to  the 
east,  as  do  those  also  on  the  whole  which  occur  at  the  southwestern  corner 
of  the  map.  But  from  the  Oomstock  west  the  country,  excepting  one  or  two 
small  masses  of  granite,  is  completely  covered  by  volcanic  rocks,  for  a 
distance  of  about  12  miles,  or  until  the  main  range  of  the  Sierra  Nevada 
is  reached.  This  grand  feature  of  the  continent  is  far  too  complex  to  be 
simply  characterized  as  an  anticlinal,  but  the  declivities  opposite  the  Com- 
STOCK  show  more  or  less  metamorphosed  strata  with  an  easterly  dip,  and 
it  is  fair  to  infer  that  for  some  distance  from  its  vast  mass,  i.  e.,  in  the  coun- 
try between  it  and  Virginia,  the  strata  underlying  the  fields  of  andesites  dip 

'  Seveu  million  tons  of  water,  the  estimated  annual  discharge,  is  about  600  feet  cube. 


HEAT  PHENOMENA. 


243 


in  the  same  sense.  If  so,  a  portion  of  the  drainage  of  the  Sierra  mu.st  reach 
great  depths  beneath  the  Washoe  District,  depths  at  which  the  tempera- 
ture must  be  very  high.  It  seems  probable  enough  that  meeting  the  fissure 
of  the  CoMSTOCK  and  the  partings  subsidiary  to  it,  the  water  thus  conveyed 
to  the  region  of  heat  rises  to  the  mines.  The  hypothetical  structure  sug- 
gested is  illustrated  in  Fig.  12. 


Fig.  ly. — Ideal  section  across  the  Virginia  Range. 


What  it  would  account  for. — lu  a  country  SO  aisturbed  by  volcanic  action  and 
so  highly  metamorphic  as  that  underlying  the  Washoe  District  probably 
is,  the  circulation  must  be  much  obstructed.  Comparatively  open  water 
channels  leading  from  the  Sierra  are  likely  to  connect  only  with  fissures 
almost  capillary  near  the  Lode,  and  vice  versa.  This  would  account  for  the 
fact  that  some  springs  in  the  mines  yield  a  steady  supply  of  water,  while  in 
other  cases  a  great  body  is  eventually  pumped  out  and  leaves  only  an  insig- 
nificant flow.  It  would  also  account  for  the  increase  of  the  heat  of  the 
water  with  the  depth,  and  its  decrease  at  considerable  distances  from  the 
Lode  and  its  accompanying  fissures;  for  if  narrow  water  channels  extend 
from  a  distant  source  of  heat  towards  a  constantly  radiating  surface,  equality 
of  temperature  can  never  result.  The  rising  currents  must  constantly  lose 
heat.  Descending  currents  will  also  be  established,  which  will,  however, 
cause  only  local  irregularities  in  the  increment  of  temperature.  Where 
great  quantities  of  water  are  drained  from  a  single  source,  the  tendency 
would  plainly  be  to  a  rise  of  temperature,  and  the  head  which  the  floods  so 
often  show  would  find  an  ample  explanation  in  the  supposed  connection 
with  channels  from  the  great  range.  No  reasoning  on  such  points,  however, 
can  be  conclusive,  for  the  opportunities  of  establishing  the  truth  of  the 
hypothesis  are  very  meager. 


244  GEOLOGY  OF  THE  COMSTOCK  LODE. 


Section   II. 
THEEMAL   SUEVEY. 

Temperature  observations. — Valuable  temperature  observations  have  been  taken 
on  four  lines  near  the  Comstock,  viz.,  in  the  Combination,  new  Yellow  Jacket, 
and  the  Forman  shafts,  and  in  the  Sutro  Tunnel.  These  observations  were 
all  made  at  freshly  exposed  points  as  the  excavations  progressed,  at  a  dis- 
tance from  all  other  workings,  and  while  not  unaffected  by  some  of  the  dis- 
turbing causes  mentioned  on  page  229,  form  a  far  more  trustworthy  guide  as 
to  the  theoretical  conditions  of  the  Lode  than  a  similar  number  of  determi- 
nations made  in  the  mines.  Each  set,  too,  was  observed  as  a  matter  of 
routine  duty,  so  that  successive  observations  must  have  been  affected  by 
nearly  constant  errors;  and  though  many  of  them  were  made  with  less  pre- 
caution than  a  physicist  would  have  employed,  their  great  number  goes  far 
toward  compensating  for  any  roughness  in  the  method.  Whoever  is  familiar 
with  the  tone  of  speculative  excitement  which  prevails  in  the  mining  i-egions 
of  the  far  West,  a  tone  but  little  in  harmony  with  scientific  research,  will 
agree  with  me  that  great  credit  is  due  to  the  officers  of  the  mines  for 
making  and  preserving  these  records.  It  would  be  well  for  the  advance- 
ment of  pure  and  applied  science  if  such  a  spirit  were  general  among  those 
whose  occupations  bring  them  in  contact  with  natural  phenomena. 

Computation  of  the  observations. — The  followiug  tables  and  diagrams  need  but 
little  explanation.  On  plotting  the  temperatures  taken  in  the  shafts,  no 
indication  of  curvature  could  be  perceived,  and  a  straight  line  was  therefore 
assumed  as  expressing  the  relation  of  temperature  to  depth. 

The  equation  of  this  line  is 

t^a  -\-hd; 

where  f  is  the  temperature  in  degrees  Fahrenheit  corresponding  to  the  depth 
d  in  feet,  and  a  and  b  are  constants  to  be  calculated.     The  computations  by 


THERMAL  SUEYET,  245 

the  method  of  least  squares  were  performed  by  Dr.  Barus  and  Mr.  Reade.' 
For  the  sake  of  comparison  they  also  computed  the  observations  made  at 
the  Rose  Bridge  Colliery,  and  I  add  the  Sperenberg  observations  with  Mr 
Heinrich's  equation.  The  Sutro  Tunnel  data  cannot  be  treated  in  the  same 
way,  for  they  show  an  unmistakably  curvilinear  locus.  A  curve  was  drawn 
empyrically  through  the  plotted  points,  no  weight  being  given  to  any  pre- 
conceived idea  of  the  character  of  the  law  of  increment.  Subtangents  were 
constructed  and  found  to  be  almost  exactly  equal;  or,  in  other  words,  it 
was  found  that  the  graphical  approximation  nearly  coincided  with  the  locus 
of  an  exponential  equation 

in  which  I  denotes  the  horizontal  distance  from  the  Lode. 

The  method  of  least  squares  is,  of  course,  applicable  to  the  computation 
of  an  equation  of  this  character,  but  the  calculation  is  so  serious  an  under- 
taking as  to  be  worth  while  only  when  a  magnificent  series  of  observations 
is  to  be  reduced.  In  the  present  case  no  exterpolation  is  desired,  and  a  de- 
termination of  the  character  of  the  curve  with  an  approximate  knowledge 
of  the  value  of  the  constants  is  sufficient  for  the  purposes  of  the  discussion. 

'The  method  of  least  squares  famishes  the  formulas 

n2d^—2d.2d    ' 

^_n.2dt^2d.2t 
n.2d^—2d.2d' 

in  which  due  preference  is  given  to  the  temperatures  corresponding  to  a  greater  depth.  The  observa- 
tions become  relatively  more  accurate  as  temperature  and  depth  increase,  and  seem  also  to  have  been 
made  with  greater  care. 


246 


GEOLOGY  OF  THE  GOMSTOCK  LODE. 


Table  I.— COMBrNATION  SHAFT.     ROCK  TEMPERATURES. 

[Observations  made  by  the  superintendent.] 
Columns  3  and  4.— Observatiooa  of  depth  and  temperature,  respectively,  as  taken. 

5  and  G. — Means  of  consecutive  sets,  of  five  observations  each,  of  depth  nnd  temperature,  respectively. 
7. — Temperature  as  calculated  from  the  constants  derived. 
8. — "Error"  or  observed  temperature  minus  the  calculated  result. 

[Depths  are  given  in  feet;  temperatures,  in  degrees  Fahrenheit.] 
a  =  66.0.     b  =  0.0252  ±  0.0007. 


No. 

Date. 

Depth 
observed. 

Temperature 
observed. 

Mean  depth 
observed. 

Mean 

temperature 

observed. ' 

Mean 

temperature 
calculated. 

Error. 

1877. 

Feet 

°F. 

1 

July  17 

1,476 

106 

a 

18 

1,479 

104 

3 

19 

1,482 

103 

4 

20 

1,485 

105 

5 

21 

1,489 

100 

1,482 

103.6 

103.4 

+  0.2 

0 

22 

1,492 

105 

7 

23 

1,495 

103 

8 

24 

1,498 

103 

9 

25 

1,501 

102 

10 

26 

1,504 

104 

1,498 

103.4 

103.8 

-  0.4 

11 

27 

1,507 

106 

12 

28 

1,510 

105 

13 

29 

1,513 

104 

14 

30 

1,516 

106 

15 

31 

1,518 

104 

1,513 

105.0 

104.1 

4-  0.9 

16 

Aug.    1 

1,520 

105 

17 

2 

1,523 

103 

18 

3 

1,524 

104 

19 

4 

1,526 

102 

20 

5 

1,528 

106 

1,S24 

104.0 

104.4 

-  0.4 

21 

6 

1,530 

103 

22 

7 

1,532 

106 

23 

8 

1,535 

104 

24 

9 

1,539 

105 

25 

10 

1,541 

106 

1,535 

104.8 

104.7 

+  0.1 

26 

11 

1,544 

104 

27 

12 

1,547 

106 

28 

13 

1,550 

105 

29 

14 

1,553 

106 

30 

15 

1,556 

103 

1,550 

104.8 

106.1 

-  0.3 

31 

16 

1,559 

102 

32 

17 

1,562 

101 

33 

18 

1,565 

im 

34 

19 

1,568 

105 

35 

20 

1,571 

106 

1,565 

104.2 

105.5 

-  1.3 

36 

21 

1,574 

106 

37 

22 

1,577 

104 

38 

23 

1,579 

105 

1 

39 

24 

1,583 

106 

1 

40 

25 

1,585 

106 

1,579 

105.4 

105.8 

-  0.4 

41 

26 

1,588 

104 

42 

27 

1,591 

106 

43 

28 

1,593 

108 

44 

29 

1,596 

108 

45 

30 

1,599 

107 

1,593 

106.6 

106.2 

+  0.4 

'  Probable  error  of  one  of  these  mean  observations  =  ±0°.5. 


HEAT  PHENOMENA. 


247 


Table  1.— COMBINATION  SHAFT.     ROCK  TEMPERATURES— Continued. 


No. 

Date. 

Depth 
observed. 

Temperature 
observed. 

Mean  depth 
observed. 

Me.in 

temperature 

observed.' 

Mean 
temperature 
calculated. 

1 
Error. 

1877. 

Feet. 

"F. 

Fee^ 

°  F. 

°F. 

°F. 

46 

Aug.  31 

1,603 

106 

47 

Sept.    i 

1,605 

108 

48 

2 

1,607 

107 

49 

3 

1,609 

106 

50 

4 

1,611 

108 

1,607 

107.0 

106.5 

+  0.5 

51 

5 

1,613 

106 

52 

6 

1,615 

110 

53 

7 

1,617 

109 

54 

10 

1,620 

107 

55 

11 

1,622 

108 

1,617 

108.0 

106.8 

+  1.2 

,      56 

12 

1,624 

109 

57 

13 

1,626 

109 

68 

14 

1,629 

107 

59 

15 

1,632 

108 

60 

16 

1,635 

108 

1,629 

108.2 

107.1 

+  1-1 

61 

17 

1,637 

106 

62 

18 

1,640 

109 

63 

19 

1,642 

105 

64 

20 

1,645 

107 

65 

21 

1,647 

106 

1,642 

106.8 

107.4 

-  0.8 

66 

22 

1,649 

108 

67 

23 

1,651 

105 

68 

24 

1,654 

110 

69 

25 

1,656 

109 

70 

26 

1,658 

107 

1,654 

107.8 

107.7 

+  0.1 

71 

27 

1,661 

108 

72 

28 

1,663 

110 

73 

29 

1,665 

107 

74 

30 

1,668 

109 

75 

Oct.      1 

1,671 

110 

1,665 

108.8 

108.0 

+  0.8 

76 

2 

1,672 

109 

77 

3 

1,675 

109 

78 

4 

1,678 

110 

79 

5 

1,681 

108 

80 

6 

1,684 

107 

1,678 

108.6 

108.3 

+  0.3 

81 

7 

1,687 

110 

82 

8 

1,690 

106 

83 

9 

1,693 

109 

84 

10 

1,696 

108 

85 

11 

1,698 

107 

1,693 

108.0 

108.7 

-  0.7 

86 

12 

1,700 

109 

87 

13 

1,703 

110 

88 

14 

1,706 

110 

89 

15 

1,709 

108 

90 

16 

1,711 

110 

1.706 

109.4 

109.0 

+  0.4 

91 

17 

1,714 

108 

92 

19 

1,717 

109 

93 

20 

1,720 

107 

94 

21 

1,723 

110 

95 

22 

1,726 

108 

1,720 

108.4 

109.4 

-  1.0 

^Probable  error  of  one  of  these  "mean"  observations  =  ±  iP.b. 


248 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  I.— COMBINATION  SHAFT.    ROCK  TEMPERATURES— Continued. 


No. 

Date. 

Depth 
observed. 

Temperature 
observed. 

Mean  depth 
observed. 

Mean 
temperature 
observed.' 

Mean 
temperature 
calculated. 

Error. 

1877. 

Feel. 

oj.. 

Fett. 

°F. 

°F. 

or. 

96 

Oct.    23 

1,  728 

109 

97 

24 

1,730 

110 

98 

25 

1,733 

110 

99 

26 

1,736 

110 

100 

27 

1,738 

111 

1,733 

110.0 

109.7 

+  0.3 

101 

28 

1,740 

110 

102 

Nov.  22 

1,744 

110 

103 

23 

1,746 

108 

104 

24 

1,748 

109 

105 

25 

1.750 

109 

1,746 

109.2 

110.0 

-  0.8 

106 

26 

1,752 

110 

107 

27 

1,754 

107 

108 

28 

1,756 

108 

109 

30 

1,758 

110 

110 

Dec.      1 

1,760 

110 

1,756 

109.0 

110.3 

-  1.3 

111 

2 

1,762 

109 

112 

3 

1,764 

110 

U3 

4 

1,766 

108 

114 

6 

1,768 

110 

115 

6 

1,770 

Ul 

1.768 

109.6 

110.6 

-  0.9 

116 

7 

1,773 

112 

117 

8 

1,T76 

110 

118 

9 

1,779 

112 

119 

10 

1,782 

113 

120 

11 

1,785 

112 

1,779 

111.8 

110.8 

+  1.0 

121 

12 

1,788 

111 

122 

13 

1,790 

U3 

123 

14 

1,793 

112 

124 

15 

1,796 

110 

125 

16 

1,798 

m 

1,793 

111.4 

111.2 

+  0.2 

126 

26 

1,800 

U3 

127 

27 

1,803 

no 

128 

28 

1,806 

112 

129 

29 

1,808 

113 

130 

30 

1,810 

112 

1,805 

112.0 

111.6 

+  0.5 

131 

31 

187& 

1,812 

UO 

132 

Feb.     1 

1,900 

113 

133 

7 

1,924 

114 

134 

14 

1,950 

114 

135 

Mar.     1 

1,986 

U6 

1,914 

U3.4 

114.2 

-0.8 

136 

15 

2,000 

118 

137 

Apr.     5 

2,070 

118 

138 

27 

2,135 

127 

139 

May  27 

2,207 

128 

140 

Jane  10 

2,230 

112 

2,128 

120.6 

119.6 

+  1.0 

'  Probable  error  of  one  of  these  "mean  "  observations  =  ±  O^.S. 


(249) 


250 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  II.— YELLOW  JACKET  SHAFT. 

lObBervations  taken  by  the  official  in  charge,  in  drill-holes  3  feet  deep;  records  Undly  fomislied  by  Capt.  Thomas  Taylor.  1 

a  =  53.1.     i)=0.  0334  ±0.0009. 


No. 


Date. 


Depth  of 
drill-hole. 


Character  of 
the  rock,     i 


Depth. 


1877. 
Aug.  28 
Aug.  30 
Sept.  11 
Sept.  14 
Oct.  27 
Oct.  30 
Not.  4 
Nov.  10 
Nov.  14 
Nov.  28 

11  I  Dec.  15 

12  I  Dec.  29 


1878. 
Jan.  20 
Fob.  15 
Mar.  22 
Apr.  1 
May  27 
June  22 
Aug.  10 
Aug.  30 
Dec.     7 


Inches. 
22 
20 
13 
15 
24 
24 
21 
24 
36 
20 
22 
15 

30 
20 
18 
36 
24 
20 
18 
15 


Wet 

"Wet. 

Wet 

Dry.. 

Dry.. 

Dry.. 

Dry  . 

Wet. 

Dry.. 

Wet. 

Wet. 

Wet. 

Wet. 

Dry-. 

Dry.. 

Wet 

Dry. 

Wet. 

Wet. 

Wet. 

Wet. 


Fett. 
845 
849 
874 


932 

945 

960 

966 

1,000 

1,054 

1,095 

1,167 
1,212 
1,316 
1,333 
1,451 
1,600 
1,660 
1,720 
2, 017 


Temperature  Temperature 
i    observed. I    I   calculated. 


°  F. 
80.0 
80.0 
79.0 
82.0 
83.0 
85.0 
85.0 
84.0 
88.0 
81.0 
89.0 
92.0 

94.0 
98.0 
95.0 
100.0 
104.0 
106.0 
108.0 
110.0 
118.0 


8L4 
81.5 
82.3 
82.6 
84.0 
84.3 
84.7 
85.2 
85.4 
86.5 
88.3 
89.7 

92.1 
93.6 
97.1 
97.6 
101.7 
106.6 
108.6 
110.6 
120.3 


Error. 


o  J?_ 

-1.4 

-1.5 

-.3.3 

-0.6 

-1.0 

+0.7 

+0.3 

-1.2 

+2.6 

-2.5 

+0.7 

+2.3 

+1.9 
+4.4 
-2.1 
+2.4 
+2.3 
-0.« 
-0.6 
-0.6 
-2.5 


•  Probable  error  of  an  observation  =  ±  1°.  4. 


:;■     I 


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I    I 

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•::•  H 

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i  ? 
o 
W 

H 
ai 

W 
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a 
o 

>- 

s^ 

CO 

W 
!>► 
■^ 

K) 
13 
W 

;> 

1-3 

d 

03 


(251) 


252 


GEOLOGY  OF  THE  COMSTOGK  LODE. 


Table  III.— FORMAN  SHAFT.    ROCK 
TEMPERATURES. 

(From  100  to  1,800  feet.] 
a  =  49.8.     5  =  0.0326  ±  0.0006. 


No. 

Depth.' 

Temperature 
observed. ' 

Temperature 
calculated. 

Error. 

Feet. 

"F. 

"F. 

OF. 

1 

100 

50.5 

53.0 

-  2.5 

2 

200 

55.0 

56.3 

-  1.3 

3 

300 

62.0 

59.6 

+  2.4 

4 

400 

60.0 

62.8 

-  2.8 

5 

500 

68.0 

66.1 

+  1.9 

6 

600 

71.5 

69.3 

+  2.2 

7 

700 

74.8 

72.6 

+  2.2 

8 

800 

76.5 

75.8 

+  0.7 

9 

900 

78.0 

79.1 

-  1.1 

10 

1,000 

81.5 

82.4 

-  0.9 

11 

1,100 

84.0 

85.6 

—  L6 

12 

1,200 

89.3 

88.9 

+  0.4 

13 

1,300 

91.5 

92.1 

-  0.6 

U 

1,400 

90.5 

95  4 

+  1.1 

15 

1,500 

101.0 

98.6 

+  2.4 

16 

1,600 

103.0 

101.9 

+  1.1 

17 

1,700 

104.5 

105.2 

-  0.7 

18 

1,800 

105.5 

108.4 

-  2.9 

Table  IV.— FORMAN  SHAFT.     ROCK 
TEMPERATURES. 

[From  50O  to  2,300  feet.] 
o  =  53.2.    5=0.0296  ±  0.0002. 


*  Probable  error  of  an  observation  =  ±  1°.3. 


No. 

Depth. 

Temperature 
observed.' 

Temperature 
calculated. 

Error. 

Feet. 

°F. 

°F. 

°F. 

5 

500 

68.0 

68.1 

—  0.1 

6 

600 

71.5 

71.0 

+  0.5 

7 

700 

74.8 

74.0 

+  0.8 

8 

800 

76.5 

76.9 

-  0.4 

9 

90O 

78.0 

79.9 

-  1.9 

10 

1,000 

81.5 

82.9 

-  1.4 

11 

1,100 

84.0 

85.8 

-  1.8 

12 

1,200 

89.3 

88.8 

+  0.5 

13 

1,300 

91.5 

9L7 

—  0.2 

14 

1,400 

96.5 

94.7 

+  1.8 

15 

1,500 

101.0 

97.6 

+  3.4 

16 

1,600 

103.0 

100.6 

+  2.4 

17 

1,700 

104.5 

103.6 

+  0.9 

18 

1,800 

105.5 

106.5 

—  1.0 

19 

1,  900 

106.0 

109.5 

—  3.5 

20 

2,000 

111.0 

112.5 

-  1.5 

21 

2,100 

119.5 

115.4 

+  4.1 

22 

2,200 

116.0 

118.4 

—  2.4 

23 

2,300 

121.0 

12L2 

—  0.2 

•Probable  error  of  an  observation  =  ±  1<>.4. 


Table  V.— FORMAN  SHAFT.    WATER  TEMPERATURES. 

o=45.8.    6=0.0373  ±0.0010. 


No. 

Depth. 

Temperature 
observed.' 

Temperatnre 
calculated. 

Error. 

Feet. 

OF. 

OF. 

°F. 

1 

40O 

62.0 

60.8 

+  1.2 

2 

500 

65.0 

64.S 

+  0.6 

3 

60O 

70.0 

68.2 

+  1.8 

4 

700 

73.0 

72.0 

+  1.0 

5 

800 

75.0 

75.7 

-  0.7 

6 

900 

77.6 

79.4 

-  1.9 

7 

1,000 

80.6 

83.2 

—  2.7 

8 

1,100 

83.0 

86.9 

—  3.9 

9 

1,200 

91.0 

90.6 

+  0.4 

10 

1,300 

94.0 

94.4 

—  0.4 

11 

1,400 

100.0 

98.1 

+  1.9 

12 

1,500 

104.0 

10L8 

+  2.2 

13 

1,600 

106.0 

105.6 

+  0.4 

^Probable  error  ot  an  observation^  ±1°.3. 
These  temperatures  were  ascertained  by  drilling  holes  not  less  than  three  feet  deep  into  the  rock  and  inserting  a  Ne- 
gretti  Si.  Zambra  slow-acting  thermometer  (of  the  pattern  adopted  by  the  Underground  Temperatnre  Committee  of  the 
British  Association  and  standardized  at  Kew)  into  the  hole,  closing  the  hole  with  clay,  and  leaving  the  thermometer  for  from 
12  to  24  hours.    Not  less  than  three  holes  were  tried  at  each  point. 


254 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  VI.— ROSE  BRIDGE  COLLIERIES,  AT  INCE,  NEAR  WIGAN. 


[Observations  given  on  tiie  authority  of  John  Arthur  PbillipB.  esq.  It  is  not  stated  whether  the  temperatures  are 
those  of  the  roclt  or  the  water.  The  original  data  for  depth,  in  fathoms,  are  contained  in  column  2.  Observations  on 
Metalliferous  Deposits  and  Subterranean  Temperatures,  by  W.  J.  Henwood,  p.  775.] 

a  =  56.0.     6  =  0.0149  ±0.0004. 


No. 

Depth. 

Bepth. 

Temperature 
observed.' 

Temperature 
calculated. 

°  F. 

Error. 

Fathoms. 

Feet. 

°  F. 

OF. 

1 

80.5 

483 

64.5 

63.2 

+  1.3 

2 

100.0 

600 

66.0 

64.9 

+  1-1 

3 

279.0 

1,674 

78.0 

80.9 

-  2.9 

4 

302.5 

1,815 

80.0 

83.0 

-  3.0 

5 

315.0 

1,890 

83.0 

84.1 

-  1.1 

6 

331.5 

1,989 

85.0 

85.6 

-  0.6 

7 

335.5 

2,013 

86.0 

86.0 

±0.0 

8 

339.5 

2,037 

87.0 

86.3 

+  0.7 

9 

367.0 

2,202 

88.5 

88.8 

-  0.3 

10 

372.5 

2,235 

89.0 

89.2 

-  0.2 

11 

380.5 

2,283 

90.5 

90.0 

+  0.5 

12 

387.5 

2,325 

91.5 

90.6 

+  0.9 

13 

391.5 

2,349 

92.0 

91.0 

+  1.0 

14 

400.0 

2,400 

93.0 

91.7 

+  1.3 

IS 

403.0 

2,418 

93.5 

92.0 

+  1.5 

1 1'robable  eiTor  of  an  observatiqu=  ±lo.O. 


o 

H 

H 
W 

S 
O 

o 
tri 

w 

>< 

>■ 

O 
§ 

n- 

21 
CO 

W 
H 


H 

q 

w 


(255) 


256 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  VH.— SPERENBERG. 


[The  observations  were  taken  -with  the  geothermometer,  and  the  column  of  water  was  cat  off  on  both  sides  (Zeitschrift 
for  B.-  H.-  and  S.-Wesen  im  preas.  Staate,  xx.,  1872,  p.  225).  In  the  third  and  foarth  colomns  the  data  are  converted  into 
terms  of  English  units  for  convenienco  in  comparing  them  with  those  obtained  at  "Washoe.] 


Depth 
in  Ehenish 

feet. 

Rock 

temperature, 
Keanmur. 

Depth 

in  English 

feet. 

Bock 

temperature, 
I'ahrenheit. 

100 

10.16 

103 

55 

300 

14.60 

309 

65 

400 

14.80 

412 

65 

500 

15.16 

515 

66 

700 

17.06 

721 

70 

000 

18.50 

927 

74 

1,100 

20.80 

1,133 

79 

1,300 

21.10 

1,339 

80 

1,500 

22.80 

1,545 

83 

1,700 

24.20 

1,751 

87 

1,900 

25.90 

1,957 

90 

2,100 

28.00 

2,163 

95 

2,300 

28.50 

2,369 

9S 

2,500 

29.70 

2.575 

99 

2,700 

30.50 

2,781 

101 

3,390 

30.15 

3,492 

113 

4,042 

38.25 

4,163 

118 

17  C  I, 


(257) 


258 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  VIIL— SUTEO  TUNNEL. 
IThe  temperatures  are  usually  the  average  of  four  observations  on  different  days  of  the  month.    Observations  taken  by 
the  surveyor.    Bock  temperatures  with  Gall  thermometer  in  regular  drill-hole;  water  temperatures  with  common  Kendall 
thermometer.] 


Date. 


1875. 

April 

May 

June 

July 

August 

September  . 

October 

November  - . 
December . . 

1876. 

January 

February    . . 

March     

April 

May 

June 

July 

August  

September  . . 

October 

November . . 
December... 


Mean  distance 

frora  east  wall 

of  Lode. 


Feet. 

10, 849 

10,  575 

10,  241 

9,883 

9,512 

9,171 

8,866 

8,556 

8,291 

8,043 
7,739 
7,505 
7,175 
6,794 
6,513 
6,262 
5,988 
5,C51 
5,326 
5,008 
4,687 


Mean  temper- 
ature of  water 
at  face. 


o  ^^ 
79 
78 
79 
82 
83 
84 
82 
84 
85 

85 
65 
84 
85 
84 
84 
84 
85 
86 
86 
87 
87 


Date. 


1877. 

January  

February 

March 

April 

May 

June 

July 

August 

September    — 

October 

November    . .   . 

I  December 

I                1878. 
i  January 

February 

March 

April 

May 

June 


Mean  distance  |  Mean  temper- 

fiom  east  wall  ature  of  water 

of  Lode.  at  face. 


Feet. 
4,329 
3,935 
3,651 
3,455 
3,154 
2,898 
2,560 
2,250 
2,052 
1,924 
1,743 
1,513 

1,275 
1,048 
818 
577 
342 
128 


o  p^ 
88 


92 
92 
93 
94 
96 
95 


108 


Mean  temper- 
ature  of    rock 
at  face. 


102 
108 
110 
111 
110 
110 


Table  IX.— COMPARISON  OF  RESULTS. 
1. t  =  a  +  bd; 


t  in  degrees  Fahrenheit. 
d  in  feet  from  top  of  shaft. 


Table. 


I 

n 
m 

IV 

V 

VI 

VII 


Mine, 


Combination  shaft 

Yellow  Jacket 

Forman  shaft,  100  to  1,800 

Forman  shaft,  500  to  2,300 

Forman  shaft,  100  to  1,800 

Rose  Bridge  collieries  

Sperenberg,  700  to  2,700  (Heinrich)" 


53 
50 
53 
46 
56 
59 


6X10' 


25 
33 
33 
30 
37 
15 
17 


Kemarks. 


Temper.itnre  of  rock. 

Do. 

Do. 

Do. 
Temperature  of  water. 
Not  known. 


2.. 


=      ,  g*.   J  Tin  degrees  centigrade, 
r      *-rPoi   ^fiin  meters  from  top  of 


shaft. 


Table. 

Mine. 

a 

PXIO' 

Bemarks. 

1 

I 

19 
12 
10 
12 
8 
13 
15 

46 
61 
59 
54 
68 
27 
31 

Temperature  of  rock. 

Do. 

Do. 

Do. 
Temperature  of  water. 
Not  known. 

n 

m 

IV 
V 
VI 

vn 

Forman  shaft,  50O  to  2,300 

Forman  shaft,  100  to  1,800       

Koae  Bridge  collieries 

Sperenberg,  700  to  2, 700  ( Hcinrich) 

>  Heinrioh's  equation  as  given  by  himself  in  Khenish  feet  and  degrees  Keaomur  is 

(  =  11.8277  +  0.0077828  d. 


n 


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260  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Reasons  for  some  fluctuations. — A  part  of  tliG  fluctuations  of  the  observatioDS  in 
the  Washoe  District  can  be  reasonably  accounted  for.  In  the  Forman 
shaft  it  will  be  observed  that  the  water  temperatures  are  somewhat  lower 
than  the  rock  temperatures  above  a  depth  of  1,160  feet.  The  upper  portion 
of  this  shaft  passes  through  decomposed,  and  in  part  disintegrated,  augite- 
andesite.  Near  its  under  surface,  however,  this  rock  is  somewhat  fresher, 
and  is  there  unusually  fine-grained  and  rhyolitic  in  structure.  It  therefore 
offers  some  resistance  to  the  rise  of  waters  from  below,  and  almost  none  to 
the  descent  of  the  slight  atmospheric  precipitation.  The  point  at  which  the 
water  grows  hotter  than  the  rock  is  exactly  that  at  which  the  shaft  passes 
from  augite-andesite  into  the  underlying  hornblende-andesite.  At  1,700 
feet  the  shaft  became  so  hot  that  it  was  necessary  to  shower  cold  water  from 
the  surface.  The  subsequent  water  temperatures  were  excluded  from  the 
calculation,  and  it  is  most  likely  that  the  rock  temperatures  were  somewhat 
affected.  This  offers  a  probable  explanation  for  the  abnormally  low  tem- 
peratures of  the  rock  immediately  below  this  point.  Mr.  Forman  informs 
me  that,  from  the  2000-foot  level  on,  the  practice  of  showering  water  into 
the  shaft  was  abandoned. 

As  may  be  seen  from  the  section  through  the  Yellow  Jacket  (Atlas-sheet 
VII.),  this  shaft  passes  through  diabase  and  mica-diorite  alternately,  and  such 
changes  are  likely  to  exaggerate  the  ordinary  disturbing  influences.  That 
portion  of  the  Combination  shaft  in  which  observations  were  taken  is  wholly 
in  diabase,  but  there  is  evidence  of  disturbed  conditions.  The  water  in  the 
face  of  the  Sutro  Tunnel  opposite  the  Combination  shaft  was  about  5° 
cooler  than  the  rock  in  the  shaft,  while  a  reverse  relation  would  have  been 
expected.  The  shaft  observations  also  fluctuate  somewhat  violently  near 
this  level,  while  for  the  interval  from  1,900  to  2,100  feet  the  increment  is 
sensibly  the  same  as  in  the  other  shafts.  It  seems  probable,  therefore,  that 
the  high  value  of  a  and  the  low  value  of  b  resulting  from  the  reduction  of 
the  observations  is  somewhat  misleading,  and  that  local  variations  of  struc- 
ture only  cause  them  to  differ  essentially  from  those  obtained  for  the  Yelloto 
Jacket  and  the  Forman  shafts. 

Conditions  in  the  Sutro  Tunnel. — It  will  probably  at  oucc  occur  to  the  reader 
that  the  depth  of  the  Sutro  Tunnel  below  the  surface  is  far  from  iiniform. 


HEAT  PHENOMEIfA.  261 

The  smallest  depth,  however,  of  that  section  of  the  tunnel,  10,000  feet  long, 
for  which  the  temperatures  are  plotted  is  above  1,000  feet,  and  for  the  last 
5,500  feet  of  the  tunnel  the  average  depth  below  the  surface  is  about  1,500 
feet,  with  comparatively  small  surface  variations.  When  it  is  considered 
that  the  annual  variation  of  temperature  commonly  ceases  to  be  perceptible 
at  a  depth  of  100  feet,  it  appears  that  the  irregularities  of  temperature  in  the 
Sutro  Tunnel  due  to  the  character  of  the  surface  topography,  above  this  last 
5,500  feet  at  least,  must  be  insensible.  The  variations  from  the  exponential 
locus  are,  no  doubt,  due  to  the  character  of  the  rock,  which,  as  is  indicated 
in  the  section  (Atlas-sheet  VI.),  shows  alternate  belts  of  greater  and  less 
decomposition.  Rock  temperatures  would  have  been  preferred  to  water 
temperatures  had  they  been  recorded,  but  such  was  not  the  case. 

Conditions  in  the  laterals. — Rock  tempcraturcs  have  been  taken  from  time  to 
time  in  the  north  and  south  lateral  branches  of  the  Sutro  Tunnel,  but  in 
large  part  these  branches  pass  close  tQ  mines  which  have  been  worked  for 
years  to  a  much  lower  level  than  that  of  the  tunnel.  The  mean  of  the 
observations  taken  in  the  south  branch  as  far  as  the  Imperial  ground  is 
almost  exactly  the  same  as  that  of  the  observations  in  the  north  branch  as 
far  as  the  Ophir,  1 13°  or  114°,  thus  confirming  the  fact,  already  well  known, 
that  within  the  limits  indicated  the  mines  are  all  hot,  and,  on  the  whole, 
pretty  nearly  equally  so.  The  north  branch  near  the  Ophir  is  three  or  four 
degrees  hotter  than  might  have  been  anticipated  from  the  observations  in 
the  main  adit,  and  the  Alpha  and  Exchequer  claims  are  nearly  as  hot.  Such 
variations  are  certainly  to  be  expected.  They  may  indicate  local  peculiar- 
ities of  structure,  such  as  the  presence  of  diagonal  fissures  leading  to  the 
Lode,  or  very  possibly  the  prevalence  of  slightly  higher  temperatures 
throughout  the  regions  lying  to  the  north  and  south  of  the  main  tunnel. 

Regularity  of  the  Forman  curve. — A  comparisou  of  thc  diagrams  shows  that  the 
observations  in  the  Forman  shaft  reveal  an  increment  not  greatly  more 
irregular  than  those  observed  at  the  Rose  Bridge  colliery,  and  at  Sperenberg. 
In  view  of  the  local  character  of  the  abnormal  temperatures  near  the  Com- 
STOCK  this  fact  is  remarkable. 

Results. — The  five  lines  of  temperatures  near  the  Lode  .form  a  tolerably 
complete  thermometric  survey,  and  justify  conclusions  of  a  definite  char- 


262  GEOLOGY  OF  THE  COMSTOCK  LODE. 

acter.  The  Forman  and  the  Yellow  Jacket  shafts  show  that  the  characteristic 
increment  is  very  close  to  1  "^  F.  for  every  33  feet  of  vertical  descent,  and, 
since  there  is  no  evidence  of  curvature,  this  rate  may  be  expected  to  con- 
tinue for  a  long  distance  below  the  present  workings.  As  the  source  of 
heat  is  approached  the  vertical  increment  must  increase,  and  the  true 
expression  for  the  relation  of  depth  and  temperature  is  probably  very  sim- 
ilar to  that  found  for  the  horizontal  increment  in  the  Sutro  Tunnel.  It 
appears  hardly  possible  that  were  the  source  of  heat  within  two  miles  of 
the  surface  no  trace  of  curvature  would  be  perceptible  in  the  diagrams  for 
a  depth  of  2,000  feet.  The  probabilities  seem  to  be  that  the  focus  is  several 
miles  from  the  surface. 

Equations  referred  to  the  datum  level. — The  cquations  for  thc  shafts  are  referred  to 
the  surface  at  the  points  where  they  are  sunk,  and  equal  values  oftidonot, 
therefore,  answer  to  the  same  level.  The  Forman  shaft  is  356  feet  and  the 
Yellow  Jacket  343  feet  below  the  datum  level  employed  in  surveys  of  the 
mines.     Referred  to  that  level,  the  equations  become: 

Forman  Shaft:  f=38-f  0.033  (i 
Yellow  Jacket :    t=A2-\-  0.033  d 

where  d  is  the  depth  below  the  datum  level. 

Correlation  with  the  tunnel  equation. — The  difference  bctwecn  the  values  of  a  in 
these  two  equations  is  4°.     Now,  the  Yellow  Jacket  shaft  is  about  2,600 
feet  from  the  croppings  of  the  Xode,  and  the  Forman  shaft  is  950  feet 
farther,  and  the  curve  obtained  from  the  Sutro  Tunnel  shows  that  such  a  k 
difference  should  exist.     Indeed,  if  in  the  equation 

^=80  +  34  e"""'^ 

—  2,600  and  —950  are  successively  substituted  for  x  the  difference  in  the 
values  of  t  obtained  will  be  4°.^  This  shows  very  clearly  that  the  law  of 
decrease  of  the  temperature  to  the  east  of  the  Lode  holds  good  for  other 
sections  than  that  taken  on  the  hne  of  the  Sutro  Tunnel;  and  this  inference 
is  strengthened  by  the  similarity  of  the  temperatures  in  the  north  and  south 

>  To  give  a  to  fractions  of  a  degree  would  manifestly  be  absurd.  If  the  fractions  resulting  from 
computation  were  to  be  retained  the  difference  in  the  value  of  a  calculated  from  the  exponential  equa- 
tion would  be  the  same  as  that  derived  from  the  observations  at  the  shafts  within  half  a  degree. 


HEAT  PHENOMENA.  263 

laterals  of  the  tunnel.  On  the  other  hand,  these  equations  give  for  the 
Sutro  Tunnel  level  (1,865  feet  below  the  datum)  temperatures  five  or  six 
degrees  higher  than  are  found  at  the  corresponding  points  in  the  adit.  This 
agrees  well  with  the  supposition  already  suggested,  that  the  isothermal 
surfaces  rise  somewhat  towards  the  south;  but  the  data  are  too  uncertain, 
and  the  rock  is  too  heterogeneous  to  warrant  applications  of  the  equations 
implying  their  absolute  accuracy.  It  appears  to  me,  under  the  conditions, 
extremely  remarkable  that  the  relations  of  temperature  to  depth  and  hori- 
zontal distance  from  the  Lode  are  capable  of  even  approximate  mathematical 
expression. 

Practical  data. — Withiu  thc  belt  of  couutry  between  2,500  and  3,500  feet 
from  the  croppings,  the  relation  of  temperature  of  the  rock  to  depth  is 
expressed  approximately  by  the  equation 

t  —  M)-\-0.()B'id, 

d  being  measured  from  the  datum  level  in  feet;  and  this  equation  may  be 
expected  to  hold  good,  with  local  fluctuations,  for  a  long  distance  below  the 
present  workings.  The  equation  gives  for  a  temperature  of  212°  a  depth 
of  5,200  feet.  The  water  will  be  found  commonly  hotter  than  the  rock, 
and  its  temperature  also  more  variable.  It  is  not  unlikely  to  be  struck  at 
a  boiling  heat  any  time  after  the  4,000-foot  level  is  passed,  and  will  in  all 
probability  be  struck  short  of  5,000  feet. 

inferencesfrom  the  Sutro  curve. — The  curve  obtained  froffi  the  observatious  made 
in  the  Sutro  Tunnel  is  clearly  a  conduction  curve,  and  proves  that  the  east 
country  is  heated  from  a  surface  at  or  near  the  Lode.  If  the  Lode  is  sup- 
posed to  have  assumed  its  present  temperature  suddenly,  the  radius  of  cur- 
vature of  this  locus  would  be  a  function  of  the  time,  and  if  the  coefficient 
of  conductivity  of  the  rock,  its  initial  temperature,  etc.,  and  all  the  condi- 
tions of  radiation  from  the  surface  were  known,  the  time  which  has  elapsed 
since  the  Lode  grew  hot  might  be  calculated.  It  is  not  likely,  however, 
that  the  temperature  of  the  Lode  has  always  been  constant  or  nearly  so, 
and  there  is  no  means  of  inferring  the  constants,  a  definite  knowledge  of 
which  would  be  necessary  to  a  mathematical  discussion  of  this  problem. 


264  GEOLOGY  OF  THE  COM8TOCK  LODE. 

But  it  is  clear  that  as  time  goes  on  the  radius  of  curvature  of  the  conduction 
curve  will  increase,  and  that  no  illimitable  time  has  elapsed  since  the  Lode 
first  assumed  a  temperature  of  above  110°  F.  on  the  1,900-foot  level. 

Results  independent  of  very  accurate  thermometers. It  is  WcU  knOWn  that  the  thermom- 
eter is  not  an  instrument  which  gives  positively  uniform  results,  and  that 
thermometric  experiments  aiming  at  a  high  degree  of  accuracy  imply  con- 
stant rerating  of  even  the  best  instruments,  which  probably  change  more 
or  less  permanently  at  each  fluctuation  of  temperature.  The  observations 
discussed  in  the  foregoing  pages  were  only  in  part  taken  with  first-class 
thermometers,  and  some  of  them  are  very  probably  affected  with  errors  of 
1°  or  even  2°  F.  from  this  cause.  This  fact,  however,  does  not  at  all  impair 
the  general  validity  of  the  results  obtained.  Suppose  the  graduation  of  the 
thermometers  employed  wholly  arbitrary,  and  that  the  graduation  of  the 
instruments  used  at  each  shaft  bore  no  relation  to  those  used  at  any  other, 
but  that  the  calibration  of  each  was  good  and  permanent  changes  in  volume 
were  absent;  the  results  obtained  would  still  show  that  the  increment  of 
temperature  from  the  surface  downward  was  affected  by  no  perceptible  law 
other  than  that  of  direct  proportionality  to  depth,  and  that  in  the  tunnel 
the  rise  of  temperature  as  the  Lode  was  approached  was  best  expressed  by 
a  geometric  ratio.  Or  suppose  that  imperfection  in  calibration  and  the  per- 
manent effects  of  expansion  induced  any  error  for  which  a  precedent  can 
be  found,  say,  even  3°  F.,  between  the  highest  and  lowest  readings  in  either 
of  the  shafts  or  the  adit ;  the  differences  themselves  are  so  large  (from  30° 
to  60°  F.)  that  the  same  general  conclusions  as  to  the  great  distance  of  the 
source  of  heat  and  the  method  of  its  communication  to  the  walls  of  the  Lode 
would  follow.  In  short,  the  indications  are  so  positive  that  no  probable 
errors  in  the  thermometers,  however  gross,  could  account  for  them  or  ob- 
scure them. 

Conclusions. — The  couutry  rock,  then,  is  heated  from  the  Lode  or  the  sys- 
tem of  fissures  closely  associated  with  it,  and  the  focus  of  this  heat  is  at  a 
vertical  distance  which  can  hardly  be  less  than  two  miles  from  the  surface, 
and  is  more  probably  four — in  short,  at  a  volcanic  distance.    Only  fluid  sub- 


HEAT  PHENOMENA.  265 

stances,  gas  or  water,  could  serve  as  a  vehicle  to  transport  this  heat  to  the 
upper  portions  of  the  Lode  ;  and  while  gas  is  absent,  the  immense  volumes 
of  hot  water  form  the  most  serious  obstacles  to  mining.  Water,  then,  has 
been  the  vehicle  of  the  heat.  The  same  results,  therefore,  as  were  arrived  at 
in  the  first  section  of  the  chapter  from  geological  and  chemical  arguments, 
are  reached  by  discussion  of  the  thermometric  observations. 


CHAPTERVITI. 
THE  LODE. 

Condition  of  the  Lode. — Dui'ing  the  period  in  which  the  field-work  for  this 
report  was  done  the  condition  of  the  Comstodk  was  not  flourishing.  The 
last  remunerative  ore  from  the  neighborhood  of  the  great  Consolidated  Vir- 
ginia and  California  bonanza  was  extracted  while  the  examination  was  going 
on,  and  no  other  body  of  similar  importance  had  been  discovered.  In  the 
course  of  the  time  covered  by  the  stoping  of  the  great  bonanza  several  small 
bodies  were  discovered  in  the  Sierra  and  Union  ground.  These,  however, 
were  speedily  worked  out.  The  old  workings  were,  with  few  exceptions, 
inaccessible,  and  the  exposures  of  the  vein  were  meager  and  unsatisfactory. 
The  study  was  therefore  necessarily  rather  one  of  the  conditions  of  the 
occurrence  of  the  great  Lode  than  of  the  vein  phenomena  in  detail.  For- 
tunately, the  attention  of  previous  investigators  took  the  opposite  direction, 
and  the  vein  has  been  amply  and  ably  described  as  far  down  as  the  large 
ore  bodies  extend.  Aided  by  former  descriptions  and  some  few  notes 
and  recollections  of  visits  made  when  several  of  the  most  important  bonanzas 
were  yielding  largely,  I  am  able  to  give  a  succinct  account  of  the  occur- 
rence of  ore  on  the  Comstock,  and  to  show  to  what  extent  the  facts  and 
theories  developed  in  the  foregoing  chapters  throw  light  on  the  structure 
observed  in  the  upper  portion  of  the  Lode,  as  well  as  upon  the  probable 
character  of  the  regions  below  those  as  yet  explored. 

General  outline. — The  surface  map,  Atlas-sheet  IV.,  shows  a  plan  of  the 
Comstock  as  it  would  appear  if  the  debris  and  talus  were  removed.^  The 
main  body  of  the  Lode  is  a  belt  of  quartz  and  vein  matter  10,000  feet  long 
and  several  hundred  feet  broad,  showing  slight  undulations  in  its  course, 

'The  scale  of  the  surface  map  is  too  small  to  show  some  of  the   minor  irregularities  iu  the  walls 
which  mjiy  he  seen  in  some  of  Mr.  King's  horizontal  sections,  or  the  outlines  of  the  horses. 
26G 


THE  LODE.  267 

but  with  a  general  strike  of  about  north  16°  east.  At  each  extremity  of 
this  main  fissure  the  Lode  ramifies  into  diverging  branches,  of  which  there 
are  two  at  the  south  end,  and  a  greater  number,  probably  more  than  are 
shown,  at  the  northern  extremity.  These  branches  dwindle  as  the  distance 
from  the  main  body  increases,  and  finall}^  disappear,  though  it  is  not  im- 
possible that  they  might  be  traced  somewhat  farther  than  the  map  shows 
them.  The  whole  system  produces  upon  the  eye  the  impression  of  a  crack 
in  slightly  elastic  material,  due  to  a  force  acting  near  the  middle  and  equal- 
ized at  the  extremities  by  dissemination  over  a  large  ai'ea.  This  impression 
is  probably  correct.  The  fissure  has  a  comparatively  constant  dip  of  from 
33°  to  45°,  though  there  are  local  irregularities  of  a  trifling  character. 

Prismatic  horse. — A  vcry  interesting  and  important  feature  of  the  Comstock, 
observable  in  cross-section,  is  the  forking  of  the  vein  at  some  distance  be- 
low the  croppings.  The  foot  wall  continues  in  typical  cases  unbroken  to 
the  surface,  but  a  secondary  fissure  rises  through  the  hanginj  wall  in  a 
more  or  less  nearly  vertical  direction,  leaving  the  foot  wall  at  a  depth  of 
several  hvindred  feet.  A  mass  of  country  rock,  which  might  be  represented 
diagrammatically  as  a  triangular  prism,  is  thus  included  within  the  external 
walls  of  the  vein.  It  is  needless  to  say  that  very  considerable  modifications 
in  the  direction,  position,  and  geometrical  form  of  the  secondary  fissure  are 
observable  in  different  portions  of  the  Lode. 

Vein  below  the  horse. — Exccptiug  iu  the  region  above  the  junction  of  the 
east  and  west  fissures,  the  vein  in  dip  is  of  very  uniform  thickness;  and 
does  not  show  as  often  or  as  prominently  as  many  lodes  the  tendency  to 
open  into  chambers  and  pinch  out  again,  which  commonly  accompanies  a 
faulting  of  one  wall  relatively  to  the  other.  This  fact  is  by  no  means  due 
to  the  absence  of  a  fault,  but  to  its  especial  character.  There  is  an  un- 
mistakable similarity  between  the  configuration  of  the  west  wall  and  that  of 
the  eastern  face  of  the  range. 

The  walls. — The  hanging  wall  of  the  Comstock  is  diabase  throughout  the 
entire  10,000  feet  of  the  main  Lode,  for  some  distance  on  the  southeast 
branch,  and  along  its  northeast  branch,  as  far  as  the  explorations  have  been 
carried.  The  east  wall  is  almost  all  in  an  extreme  state  of  decomposition  so 
far  as  the  bisilicates  are  concerned,  and  the  feldspars  also  are  frequenti}' 


268  GEOLOGY  OF  THE  COMSTOCK  LODE. 

replaced  by  alteration  products.  The  foot  or  west  wall  of  the  main  fissure 
is  granular  diorite  for  more  than  three-quarters  of  its  length,  but  at  the 
southern  end  it  is  chiefly  composed  of  metamorphic  slates.  The  foot  wall  is 
much  less  altered  than  the  hanging.  The  northern  branches,  excepting  the 
most  easterly  one,  are  inclosed  in  porphyritic  diorites,  though  stringers  of 
diabase  also  make  their  appearance  in  one  or  two  spots  on  the  fissure  which 
extends  toward  the  Utah  shaft.  The  southern  branches  pass  along  a  variety 
of  contacts. 

Black  dike. — Accompanying  the  vein  for  about  half  its  length  is  the  nar- 
row dike  of  younger  diabase  called  "black  dike."  It  is  found  only  a  little 
north  of  the  middle  of  the  main  Lode,  extending  thence  southward  and 
following  the  southwest  branch.  It  usually  lies  directly  upon  the  foot  wall, 
but  occasionally  passes  a  short  distance  behind  it.  In  the  higher  levels  it 
was  so  decomposed  as  to  be  unrecognizable  as  diabase. 

Contents  of  the  vein. — The  coutcnts  of  tho  veiu  are  simple,  on  the  whole,  con- 
sisting of  country  rock  in  fragments  varying  in  size  from  that  of  a  grain  of 
sand  to  horses  thousands  of  feet  in  length,  clay,  quartz,  and  argentiferous 
minerals.  The  quantity  of  calcite,  except  in  the  Justice,  is  wholly  insignif- 
icant, and  gypsum,  zeolites,  etc.,  are  rare.  Some  of  the  quartz  is  said  to 
contain  no  silver  or  gold:  but  for  the  most  part  it  carnes  both,  though  in 
varying  quantities.  That  which  lies  upon  or  is  inclosed  in  diorite  carries 
gold,  but  little  silver;  very  little  of  this,  however,  will  pay  the  expense  of 
extraction  and  treatment.  The  quartz  associated  with  the  hanging  wall 
carries  more  silver,  accompanied  by  gold  of  a  value  nearly  equal  to  that  of 
the  silver.^  The  variation  in  the  tenor  of  the  quartz  is  extreme,  as  it  usually 
is  in  silver  veins;  and  it  is  only  in  certain  spots  that  the  quartz  assays  above 
the  fifteen  or  twenty  dollars  necessary  to  warrant  extraction  at  the  present 
prices  of  labor  and  supplies;  while  occasionally  the  value  per  ton  of  com- 
paratively small  masses  runs  up  to  several  thousand  dollars.  Masses  of  ore 
which  will  pay  for  extraction  are  called  throughout  the  region  west  of  the 
Rocky  Mountains  bonanzas,  a  Mexican  mining  term  which  avoids  the  ambi- 
guity of  the  English  term  ore.  The  bonanzas,  therefore,  do  not  represent  by 
any  means  all  of  the  quartz  which  carries  a  perceptible  amount  of  precious 

■See  table  of  the  proportions  of  gold  and  silver  in  Comstock  bullion,  p.  9. 


THE  LODE.  269 

metals,  and  are  often  surrounded  by  low-grade  ores  in  great  quantities. 
Though  there  are  exceptions  to  the  rule,  large  bodies  of  quartz  commonly 
contain  bonanzas.  The  occurrence  of  these  bodies  depends  on  very  com- 
plex conditions,  and  no  attempt  can  be  made  to  account  for  their  position 
until  the  sections  of  the  Lode  have  been  passed  in  review.  With  two  very 
important  exceptions  they  have  all  been  found  in  the  secondary  fissure,  not 
on  that  with  a  constant  dip.  Excepting  the  Justice  body  they  have  all 
occurred  in  contact  with  the  east-country  diabase. 

Complex  structure  of  the  comstock. — The  Ordinary  couccptlon  of  a  vein  is  a  simple 
crack  in  the  earth's  crust  charged  with  ore  and  gangue.  The  Comstock  does 
not  realize  this  conception  even  approximately.  With  the  possible  exception 
of  the  east-and-west  veins  near  Silver  City,  the  whole  fissure  system  of  the 
District  is  referable  to  a  single  mechanical  cause  and  the  charging  of  the 
fissures  is  in  all  probability  due  to  simultaneous  lixiviation.  The  branches 
of  the  Lode  to  the  north  and  south  are  structurally  integral  portions  of  the 
Comstock,  but  the  Lode  considered  as  a  great  ore  deposit  is  limited  to  the 
contact  of  the  diabase  with  the  underlying  rocks. 

Cross-section  through  the  c.  &  c. — Thc  ffiost  iutcrestiug  vcrtlcal  cross-section  of 
the  Lode  is  that  through  the  C.  &  C,  Consolidated  Virginia,  and.  Andes  shafts; 
and  fortunately  this  was  pretty  thoroughly  accessible  at  the  time  of  examina- 
tion. The  foot  wall  is  diorite,  and  the  hanging  wall  substantially  diabase, 
while  the  surface  is  capped  with  earlier  hornblende-andesite.  The  secondary 
fissure  at  this  point  was  not  simple  but  multiform,  splitting  the  wedge  of 
country  rock  into  sheets  or  sharper  wedges.  The  intervening  space  is  filled 
with  quartz,  none  of  which  has  been  stoped  on  the  plane  of  this  section, 
though  remunerative  ore  has  been  extracted  in  the  Andes  at  a  short  distance 
from  it,  and  a  very  important  ore  body  occurred  near  the  surface  some  500 
feet  to  the  north.  The  quartz  contains  numerous  fragments  of  country  rock, 
too  small  to  be  shown  in  the  drawing;  and  some  of  the  horse  is  so  silicified 
as  to  be  regarded  in  mining  as  quartz.  At  400  feet  from  the  surface  the 
different  fissures  unite,  and  the  main  fissure  is  supposed  to  continue  without 
interruption  to  the  bottom  of  the  Consolidated  Virginia  shaft,  where  it  is  a  mere 
crack.  Why  the  vein  has  not  been  prospected  for  an  interval  of  about 
1,200  feet  I  cannot  say.     The  great  bonanza  which  has  yielded  over  one- 


270  GEOLOGY  OF  THE  COMSTOOK  LODE. 

third  of  the  product  of  the  whole  Lode  stands  in  a  vertical  position  and  ex- 
tends 500  feet  from  the  fissure.  Below  it  large  masses  of  diorite  are  embed- 
ded in  indeterminable  vein-matter  and  diabase. 

In  the  funnel-shaped  mass  directly  under  the  croppings  a  notable  feature 
is  the  variation  in  the  character  of  the  quartz.  This  is  hard  and  firm  where 
it  lies  upon  the  west  wall,  and  so  far  from  it  as  the  general  structure  indi- 
cates that  the  quartz  sheets  are  parallel  to  the  line  of  the  main  fissure. 
East  of  the  horse,  on  the  other  hand,  the  quartz  is  in  great  part  crushed  to 
a  condition  like  that  of  commercial  salt.  The  horse-matter  in  this  portion 
of  the  section  is  also  accompanied  by  heavy  clays.  The  ore  near  the  crop- 
pings in  this  region  was  heavily  charged  with  galena,  blende,  and  pyrite, 
differing  in  this  respect  from  the  great  bonanza  in  the  same  vertical  plane, 
and  from  the  principal  ore  bodies  of  the  Lode. 

The  "great  bonanza.' — Tlic  bouauza  cousistcd  of  a  group  of  three  bodies,  one 
of  them  far  larger  than  the  others,  and  one  of  very  small  dimensions.  The 
cross-section  under  discussion  and  the  longitudinal  vertical  projection,  Atlas- 
sheet  X.,  give  a  better  idea  of  the  geometrical  form  and  the  position  of  this 
important  group  than  any  description  could  do.^  It  was  composed  of 
crushed  quartz,  including  fragments  of  country-rock,  and  carried  a  few 
h  M'd,  narrow,  vein-like  seams  of  very  rich  black  ores,  consisting  of  ste- 
phanite  and  similar  minerals,  while  nearly  the  whole  mass  of  "sugar-quartz" 
was  impregnated  to  a  moderate  extent  with  argentite  and  gold,  the  latter 
probably  in  a  free  state.  The  immense  volume  of  these  soft  ores  more  than 
compensated  for  their  moderate  tenor,^  and  much  the  greater  part  of  the 
entire  yield  of  the  bonanza  was  derived  from  them.  They  carried  a  mod- 
erate amount  of  pyrite.  A  great  part  of  the  space  stoped  out  consisted  of 
fragments  of  country-rock,  impregnated,  however,  with  ore,  and  assaying 
well.  These  fragments  were  highly  decomposed,  but  perfectly  recognizable 
by  their  green  color  and  traces  of  porphyritic  structure.  They  were  not 
rounded,  and  I  never  saw  traces  of  the  concentric  structure  which  any  pro- 
cess of  replacement  must  have  imparted  to  them.  On  the  contrary,  they 
were  as  sharply  defined  as  if  freshly  broken.    Comb  structure  was  not  visible 

'Mr.  Church  gives  excellent  illustrations  of  the  form  of  this  body  on  difi'erent  levels,  but  the 
black  rock  west  of  the  bonanza  is  not  black  dike. 

"The  ore  of  the  great  bonanza  averaged  abi>ut  .$80  per  ton,  but  this  included  the  rich  stringers. 


the:  lode.  271 

in  the  bonanza  on  a  large  scale,  but  where  masses  of  country-rock  were 
favorably  placed,  the  space  between  them  often  showed  this  peculiarity, 
indicating  that  the  fragments  had  acted  as  centers  of  crystallization  for  the 
quartz.  The  same  appearance  was  noticed  by  Mr.  King  in  the  earlier 
bonanzas.  Clays  were  by  no  means  a  prominent  feature  of  this  body, 
though  not  absent.  The  endless  sheets  of  clay  following  and  intersecting 
the  ore  bodies  which  were  so  striking  in  the  upper  levels  throughout  the 
Lode  seem  to  have  disappeared  below  the  junction  of  the  fissures.  To  the 
east  of  the  bonanza,  especially  in  the  region  exposed  by  the  north  branch 
of  the  Sutro  Tunnel,  the  rock  is  very  heavily  charged  with  pyrite,  as  well 
as  greatly  decomposed;  and  the  sulphuret  is  clearly  formed  within  the 
augite  crystals  of  the  diabase.  The  dioritic  masses  east  of  and  below  the 
bonanza  are  shattered  and  somewhat  decomposed,  but  not  to  the  same 
extent  as  the  augitic  rock  The  material  laid  down  as  "vein-matter"  on  this 
and  the  other  sections  is  crushed  rock,  so  highly  altered  that  its  original 
character  cannot  be  determined  with  certainty.  The  color  underlying  the 
conventional  markings  which  designate  vein-matter  indicates  what,  in  my 
opinion,  is  its  probable  lithological  origin. 

Inferences  from  the  c.  &  c.  section. — It  is  SO  difficult  to  retain  detailed  descriptions 
in  the  memory,  that  it  seems  advisable,  in  the  interest  of  the  reader,  to  draw 
such  inferences  from  each  section  as  are  justifiable,  without  waiting  till  they 
have  all  been  passed  in  review.  The  occurrence  of  the  secondary  fissures 
on  the  CoMSTOCK  appeared  to  Baron  v.  Eichthofen  clear  evidence  that  the 
surface  had  not  undergone  great  erosion  since  tlie  formation  of  the  vein.  Mr. 
King  concurred  in  this  opinion,  and  it  also  appears  to  me  essentially  a  sur- 
face phenomenon ;  for  had  the  east  wall  near  the  present  top  of  the  fissure 
been  backed  up  by  thousands  of  feet  of  rock,  it  is  difficult  to  see  how  it 
could  possibly  have  yielded  in  the  manner  shown  by  the  section.  The 
secondary  fissures  must,  too,  have  been  caused  by  faulting  action,  for  in  no 
other  way  can  a  tendency  to  rupture  in  a  vertical  direction  be  accounted 
for.  That  the  east  country  throughout  the  mines,  and  prominently  in  this 
neighborhood,  shows  numerous  signs  of  faulting,  has  already  been  explained 
at  length,  as  well  as  that  the  sheeted  structure  is  not  ascribable  to  eruptive 
bedding. 


272  GEOLOGY  OF  THE  COMSTOCK  LODE. 

sugar-quartz. — The  microscope  further  shows  that  the  sugar-quartz  is  com- 
posed of  crushed  crystals/  and  this  can  also  be  demonstrated  macroscopi- 
cally.  In  interstices  between  fragments  of  country  rock,  bunches  of  quartz 
crystals  are  not  uncommon,  and  these  though  fractured  are  sometimes  held 
together  by  the  support  of  the  surrounding  material.  In  such  cases  the  same 
crack  can  sometimes  be  observed  running  through  a  considerable  number  of 
crystals,  proving,  if  necessary,  that  they  have  not  yielded  to  an  internal 
stress,  but  to  an  external  force.  Though  the  whole  country  is  greatly 
broken  up,  so  that  the  average  size  of  the  blocks  of  country  rock  showing 
no  fissures  is  not  much  above  the  size  of  a  man's  fist,  it  is  nowhere  reduced 
to  the  fineness  of  sugar-quartz.  This  need  cause  no  surprise,  however,  for 
miners  and  mill  men  are  well  aware  that,  in  spite  of  its  hardness,  quartz  is 
very  readily  crushed,  far  more  readily  than  volcanic  rocks,  or  even  than 
limestone.  The  occurrence  of  sugar-quartz,  then,  is  an  evidence  of  move- 
ment, and  this  can  have  taken  place  only  in  one  direction,  that  of  the  dip 
of  the  fissure ;  for  even  if  it  were  conceivable  that  the  whole  country  in  this 
neighborhood  might  be  compressed  latterly,  the  behavior  of  the  quartz  in 
the  upper  levels  would  prove  the  supposition  inapplicable.  The  quartz- 
sheets  which  are  parallel  to  the  fissure  are  solid,  or,  at  most,  according  to 
Mr.  King,  show  a  slaty  structure ;  while  the  masses  which  are  not  parallel 
to  the  fissure  are  crushed.  In  some  of  the  bonanzas  in  other  portions  of 
the  CoMSTocK,  Mr.  King  noticed  a  parallelism  to  the  Lode  even  in  the 
crushed  masses,  and  such  a  phenomenon  is  also  said  to  have  been  observed 
in  the  great  bonanza  of  this  section. 

Period  of  the  fault. — Siucc  tlic  sccoudary  or  east  fissure  was  filled  with  quartz, 
the  faulting  action  to  which  the  existence  of  this  fissure  is  due  must  have 
preceded  the  deposition  of  ore  on  the  Comstock;  and  since  the  ore  was 
crushed  by  a  movement  in  the  direction  of  the  dip  of  the  main  fissure,  fault- 
ing must  also  have  succeeded  the  deposition  of  ore.  The  faulting  action 
studied  in  Chapter  IV.  must  therefore  have  embraced  the  whole  or  nearly 
the  whole  of  the  period  during  which  the  deposition  of  ore  was  taking  place, 

'  The  finest  portions  of  tho  sugar-qu.irtz  mounted  in  balsam  and  examined  in  polarized  light  nnder 
the  microscope  are  unmistakably  anisotropic,  and  while  portions  of  crystal-faces  are  occasionally  visible, 
most  of  the  surfaces  are  conclioidal  fractures.  I  have  met  with  no  evidence  that  any  of  the  solid  quartz 
of  the  Comstock  has  resulted  from  the  consolidation  of  sugar-quartz,  either  by  pressure  or  any  other 
agency. 


THE  LODE.  •  273 

though  movements  may  have  occurred  only  at  long  intervals  It  is  pos- 
sible that  the  seams  of  rich  ore  in  the  great  bonanza  represent  a  deposition 
posterior  to  the  final  cessation  of  movement. 

Tenor  of  the  ore. — Tho  Variation  in  the  tenor  of  ore  is  probably  ascribable  to 
two  causes.  The  general  poverty  and  the  auriferous  character  of  the  quartz 
associated  with  the  diorite  are  probably  due  to  the  composition  of  that  rock, 
which  in  this  locality  nowhere  secretes  argentiferous  ores.  On  the  other 
hand,  the  fluctuations  in  the  composition  of  the  ore  associated  with  diabase 
are  most  likely  due  to  a  combination  of  chemical  and  dynamical  causes. 
Whatever  may  have  been  the  actual  solubility  of  the  silica  and  the  argen- 
tiferous compounds  of  the  diabase,  under  the  conditions  which  prevailed 
when  the  solutions  were  formed,  it  is  in  the  highest  degree  unlikely  that  it 
was  the  same.  When,  by  a  renewed  movement  of  the  hanging  wall,  fresh 
material  was  exposed  to  solution,  either  the  silica  or  the  silver  would  dissolve 
with  greater  relative  rapidity  than  after  prolonged  exposure  to  the  solvent 
action;  and  the  ore  deposited  would  vary  correspondingly.  It  is  also  by  no 
means  impossible  that  some  of  the  richer  ores  have  been  redeposited,  form- 
ing at  the  expense  of  surrounding  bodies  of  lower  grade. 

Indistinctness  of  the  east  wall. — The  east  wall  is  Very  iudistiuct  on  this  and  on 
most  of  the  other  sections.  This  is  in  accordance  with  the  lateral-secretion 
hypothesis.  As  has  been  seen,  the  fragments  of  country-rock  certainly  act 
as  centers  of  crystallization,  and,  had  the  solutions  risen  from  great  depths 
along  the  fissure,  quartz  must  also  have  crystallized  from  both  walls  equally; 
but  if  the  solutions  percolated  from  the  east  into  the  fissure,  this  structure 
would  certainly  not  have  resulted  unless  they  passed  the  wall  very  gradu- 
ally and  gently. 

Clays.— The  clays  of  the  Comstock  appear  to  be  for  the  most  part  mere 
attrition  mixtures  of  decomposed  but  not  necessarily  of  kaolinized  rock,  as 
has  been  explained  in  Chapter  VI.  In  this  section  it  is  observable  that  the 
horses  near  the  croppings  end  downwards  in  clay  sheets,  and  that  the  days 
are  most  abundant  where  horse  matter  lies  across  the  general  direction  of 
movement. 

Quartz  deposited  in  openings. — The    substitutiou  hypothcsis  of  oro    doposition 

receives,  as  has  been  seen,  no  support  either  from  observation  or.  theory. 
18  o  L 


274  GEOLOGY  OF  THE  COMSTOCK  LODE. 

It  appears  to  me  necessary,  therefore,  to  suppose  that  quartz  and  ore  have 
been  deposited  in  openings.  The  space  occupied  b)''  the  bonanza  can  of 
course  never  have  been  an  vininterrupted  cavern,  but  it  would  seem  to  have 
been  a  space  loosely  filled  hj  fragments  of  country  rock,  which  are  now 
rejDresented  by  the  included  horse  matter.  Though  the  country -rock  is  so 
greatly  fractured,  a  space  of  this  kind  is  by  no  means  impossible.  If  a 
large  opening  were  to  be  made  anywhere  in  the  diabase,  fragments  would 
immediately  fall  from  the  sides  and  roof.  The  latter  would  assume  the 
shape  of  a  dome,  and  though  a  complete  arch  of  blocks  would  not  form,  a 
portion  of  the  weight  of  the  overlying  country  would  be  distributed  later- 
ally, and  the  diminished  pressure  would  most  likely  be  insufficient  to  crush 
the  displaced  fragments.  The  lenticular  mass  of  diorite  below  the  bonanza 
does  not  appear  to  be  in  place.  It  was  probably  partly  separated  from  the 
west  wall  at  the  diabase  eruption,  and  since  that  time  it  seems  to  me  to 
have  moved  downwards.  Owing  to  the  irregularity  in  the  walls  consequent 
upon  its  presence,  and  to  the  difference  between  its  resistance  and  that  of 
the  diabase  to  lamination  by  faulting,  it  left  a  rent  in  the  hanging  wall, 
which  has  afi'orded  an  opportunity  for  the  deposition  of  quartz  in  the  man- 
ner just  explained. 

Cross-section  through  the  Tunnel. — The  next  sectiou  south  of  the  C.  (&  C.  is  that 
through  the  Sutro  Tunnel  and  the  Savage  shaft.  It  fails  to  cross  any  ore 
but,  as  may  be  seen  from  the  longitudinal  vertical  projection,  it  nevertheless 
passes  through  nearly  the  lowest  point  of  a  fan-like  group  of  bonanzas, 
the  "Virginia  group,"  as  it  is  often  called,  extending  from  the  Chollar  to  the 
Gould  d  Curry.  On  this  plane  the  secondary  fissure  leaves  the  west  wall  at 
a  lower  point  than  in  any  other  portion  of  the  Lode,  and  all  of  the  bonan- 
zas were  found  in  the  secondary  fissure.  Throughout  this  portion  of  the 
Lode  the  east  and  west  fissui'es  display  the  same  general  characteristics  as 
at  and  near  the  Andes.  The  west  quartz  was  hard,  according  to  Mr.  King, 
while  the  eastern  quartz,  as  I  have  myself  been  able  to  observe,  is  crushed. 
The  great  horse  body  is  split  by  quartz-masses,  which  are  not  continuous, 
however,  thinning  out  in  the  strike  and  being  replaced  by  others.  Clay 
seams  are  very  heavy  and  intersect  as  well  as  follow  the  horses.  The  ore 
was  not  "base,"  and  much  of  it  was  extraordinarily  rich.     The  bonanzas 


THE  LODE.  275 

were  very  thin  perpendicularly  to  the  plane  of  the  Lode  as  compared  with 
that  previously  described,  and  hence  occupy  a  much  greater  space  on  the 
vertical  longitudinal  projection.  In  detail  the  structure  of  these  bodies  was 
excessively  complicated,  as  may  be  seen  from  Mr.  King's  report.  It  is  not 
in  my  power  to  add  anything  to  his  description,  to  which  the  reader  is 
referred  for  more  detailed  information. 

Virginia  group  of  ore  bodies. — The  Virginia  group  of  bonanzas  lies  in  an  undu- 
lation of  the  west  wall,  the  general  shape  of  which  may  be  clearly  traced  on 
the  surface  map;  but  by  inspection  of  the  horizontal  section  on  the  Sutro 
Tunnel  level  it  will  be  perceived  that  this  depression  has  flattened  so  as  almost 
to  disappear  at  a  vertical  distance  of  about  1,900  feet  from  the  datum  point. 
Before  the  walls  were  disturbed  in  their  relative  positions,  a  solid  mass  of 
diabase  lay  in  this  local  depression.  The  fact  that  the  depression  is  limited 
to  the  neighborhood  of  the  surface  must  have  brought  an  extraordinary  strain 
to  bear  upon  the  tongue  of  east  country  rock  lying  within  it  when  the  fault 
took  place.  The  lines  of  secondary  fracture,  instead  of  I'unning  nearly  paral- 
lel to  the  Lode,  appear  also  to  have  crossed  the  continuous  prismatic  horse  so 
often  referred  to,  and  to  have  reached  the  foot  wall  at  the  extremities  of  the 
undulation.  The  mass  thus  separated  would  be  canted  eastwards  by  the 
same  force  which  effected  the  separation,  and  between  it  and  the  main  body 
of  the  east  country  there  would  form  a  crescentic  opening,  the  points  of 
which  would  he  at  the  croppings  on  the  west  wall,  while  its  greatest  width 
would  also  be  on  the  west  wall  at  the  bottom  of  the  tongue  of  east  country. 
From  the  west  wall  vertically,  or  in  the  direction  of  the  secondary  fracture, 
the  opening  would  everywhere  taper,  ending  in  a  mere  line  at  the  surface 
or  more  probably  somewhat  below  the  surface,  since  the  crushing  stress  in 
an  east-and-west  direction  would  be  powerful.  This  opening  once  formed 
would  be  immediately  blocked  by  fragments  of  rock,  and  could  never  close. 

Such  I  conceive  to  have  been  the  nature  of  the  case  in  the  region  of  the 
Virginia  group,  modified  in  detail  by  more  or  less  important  irregularities 
of  structure ;  and  it  will  be  observed  from  the  Tunnel  section  that  the  west 
quartz  tapers  from  the  surface  downward,  while  the  east  quartz  thickens ; 
showing  that  the  horse  has  revolved  slightly  on  a  horizontal  north-and-south 
axis,  remaining  firmly  in  contact  with  the  east  wall  at  the  top  and  with  the 


276  GEOLOGY  OF  THE  COMSTOOK  LODE. 

west  wall  at  the  bottom.  By  inspection  of  the  longitudinal  vertical  projec- 
tion and  of  the  mine  maps,  it  will  also  be  perceived  that  the  ore  bodies  lay- 
within  such  a  space  as  is  suggested  by  consideration  of  the  probable  results 
of  faulting. 

The  occurrence  of  the  rocks  in  the  Sutro  Tunnel  has  already  been  suf- 
ficiently discussed  in  Chapter  V.  The  various  belts  of  decomposition  indi- 
cated have  all  been  located  as  veins  upon  the  surface ;  but  there  is  nothing 
in  this  section  to  indicate  any  hope  of  ore  away  from  the  Comstock,  except 
upon  the  Occidental  lode. 

Cross-section  through  the  H.  &  N. — Tho  Httlc  (&  NoTcross  scction  passBS  through 
the  edge  of  the  largest  bonanza  of  the  Virginia  group.  Its  thinness,  com- 
pared with  the  Consolidated  Virginia  and  California  bonanza,  is  striking,  but 
would  be  somewhat  less  so  were  the  plane  of  the  section  nearer  the  axis  of 
the  body.  The  structure  of  the  horse  is  much  less  regular  than  on  the  Sutro 
section,  but  it  is  again  noticeable  that  the  western  quartz  diminishes  in 
width  as  the  depth  increases,  while  the  openings  at  the  east  increase.  The 
horse  is  intersected  by  a  nearly  vertical  quartz  body.  In  the  Chollar  these 
two  eastern  fissures  come  together.  The  black  dike  makes  its  appearance 
in  this  section,  and  is  found  running  into  the  Savage,  but  no  farther  north, 
nor  is  it  known  to  reach  the  surface  at  any  point.  The  andesite  contact  is 
laid  down  from  inferences  drawn  chiefly  from  observations  made  at  the 
Savage,  700  feet  farther  north,  the  Santa  Fe  adit  being  closed.  Most  of  the 
lower  workings  of  the  Hale  dt  Norcross  were  also  inaccessible  at  the  time 
of  the  examination,  and  it  is  not  impossible  that  the  vein  is  drawn  somewhat 
wider  than  a  careful  examination  would  justify. 

Cross-section  through  the  Jacket. — In  the  Iviperlal  grouud  the  diorite  swings  to 
the  west,  leaving  metamorphic  slates  with  an  easterly  dip  as  the  foot  wall 
in  the  Gold  Hill  mines. 

But  a  small  portion  of  the  Yellow  Jacket  workings  was  accessible  at  the 
time  of  the  investigation;  but  a  preliminary  examination  of  the  lower  levels 
had  been  effected  before  the  Gold  Hill  mines  were  flooded,  and  an  excellent 
collection,  kept  by  the  company  while  sinking  the  new  shaft,  supplemented 
by  visits  to  the  accessible  tank  stations,  furnished  all  the  necessary  informa- 
tion concerning  the  eastern  portion  of  the  section.     The  old  workings  had 


THE  LODE.  277 

been  carefully  examined  by  Mr.  King's  party,  and  the  information  recorded 
by  him,  with  additional  facts  from  the  surface,  and  from  a  few  levels  below 
the  bottom  of  the  old  shaft,  make  the  section  fairly  satisfactory. 

Several  masses  of  micaceous  diorite  crossing  the  new  shaft  are  repre- 
sented as  embedded  in  diabase.  The  evidence  already  adduced  of  the  rela- 
tive age  of  these  two  rocks  precludes  the  supposition  that  these  bodies  can 
be  intrusive,  and  the  only  tenable  supposition  seems  to  be  that  they  are  frag- 
ments detached  and  moved  into  their  present  position  by  the  diabase  erup- 
tion. That  such  an  event  is  quite  possible  is  evident,  the  wonder  being 
that  it  is  not  of  more  frequent  occurrence  on  the  Comstock. 

Fissure  dipping  west. — A  vcry  pecuHar  phenomenon  is  the  occurrence  of  an 
ore  body  in  the  Yellow  Jacket  dipping  west  and  ending  abruptly  on  the 
west  wall.  The  following  is  suggested  as  a  possible  solution.  The  earlier 
hornblende-andesite  cap  is  in  this  region  of  considerable  thickness,  and  its 
under  and  upper  surfaces  seem  to  be  nearly  parallel,  while  the  diabase 
contact  slopes  at  an  angle  of  some  33°.  The  direction  of  the  faulting 
movement  was  at  least  as  nearly  vertical  as  that  of  this  contact.  To  this 
movement  the  tenacity  of  the  andesite  offered  a  resistance,  but  as  it  con- 
tained no  parting  in  the  direction  of  motion  it  yielded  in  the  direction  of 
least  resistance,  or  nearly  at  right  angles  to  the  surface.  This  action  gave 
rise  to  the  fissure  dipping  westward.  As  the  faulting  movement  continued, 
a  second  eastern  fracture  formed  exactly  as  in  the  Virginia  mines. 

Cross-section  through  the  Belcher. — The  Belclier  sectioii  IS  madc  out  from  fcwer 
data  than  most  of  the  others,  in  spite  of  the  fact  that  the  ore-bearing  levels 
were  open  to  inspection.  No  galleries  have  been  run  into  the  east  wall  on 
this  plane,  and  there  are  no  workings  where  the  croppings  should  appear. 
The  quartz  is  continuous  on  the  slope  of  the  main  fissure  above  its  junc- 
tion with  the  secondary  fracture,  but  how  far  is  not  known.  I  believe, 
however,  that  the  fissure  might  be  followed  to  the  surface,  though  it  is 
improbable  that  ore  in  any  quantity  would  be  found.  From  the  sketch 
map.  Fig.  1,  it  appears  that  the  evidences  of  solfataric  action  run  high  up 
Crown  Point  ravine,  and  back  of  the  Belcher;  and  the  decomposition  seems 
almost  necessarily  to  indicate  a  structural  connection  between  the  surface 
and  the  deep-seated  fissures.     The  secondary  fissure  appears  to  represent 


278  GEOLOGY  OF  THE  COMSTOCK  LODE. 

the  east  fissure  of  the  Yellow  Jacket,  the  west  fissure  here  coinciding  with  the 
slope  of  the  Lode. 

The  fault  at  the  Belcher. — There  is  uiuch  Icss  evidence  of  faulting  at  this  sec- 
tion than  on  any  of  the  preceding.  The  topography  does  not  show  a 
logarithmic  character;  the  lamination  of  the  surface  rocks  is  not  percepti- 
ble, nor  is  there  much  evidence  of  such  a  structure  in  the  mine;  and  far 
more  of  the  ore  was  solid  or  composed  of  bunches  of  large  interlocked  quartz 
crystals,  with  spaces  between  them,  than  in  the  Virginia  mines.  There  is 
some  crushed  quartz,  however,  and  the  character  of  the  bonanza,  which  was 
largely  made  up  of  angular  fragments  of  country-rock,  seems  to  indicate 
faulting,  though  of  a  less  violent  and  extensive  character  than  that  which 
occurred  on  the  flank  of  Mount  Davidson.  The  bonanzas  hitherto  described 
appear  to  have  filled  spaces  due  to  secondary  fracturing,  while  that  in  the 
Belcher  seems  to  have  occupied  an  opening  due  to  changes  in  dip,  combined 
with  a  relative  movement  of  the  walls,  concave  surfaces  being  brought  into 
opposition.  An  inspection  of  the  section  can  hardly  fail  to  produce  this  im- 
pression and,  if  it  be  a  fact,  it  furnishes  another  proof  of  the  comparative 
gentleness  of  the  faulting  action  in  this  locality.  Since  the  dislocating  force  is 
manifestly  dissipated  at  the  ends  of  the  Lode  by  distribution  over  a  large 
area,  it  is  likely  to  grow  less  intense  as  the  extremities  are  approached.  The 
diminution  indicated  at  the  south  end  of  the  main  Lode  is  greater  than 
at  the  north  end. 

Small  stringers  of  good  ore  have  been  met  on  the  3, 000 -foot  level  of 
the  Belcher,  the  deepest  level  yet  reached. 

Cross-section  through  the  Forman  shaft. — The  scction  through  the  Baltimore  and 
Forman  shafts  is  more  valuable  as  a  study  of  the  succession  of  the  rocks 
than  for  any  positive  information  it  furnishes  regarding  the  Lode.  The 
contacts  in  this  portion  of  the  country  are  much  more  numerous  than  near 
Viro-inia,  and  one  of  these,  seemingly  continuous  with  the  main  Comstock 
fissure,  has  been  sufficiently  opened  to  admit  the  deposition  of  quartz.  The 
dynamical  action  must  have  been  very  slight,  however,  for  thei'e  are  no 
certain  evidences,  either  in  the  shape  of  croppings  or  of  Hues  of  profound 
decomposition,  that  fissures  from  the  surface  connect  with  this  contact  in 
depth.     But  croppings  reappear  just  below  the  Justice,  and  the  surface  and 


THE  LODE.  279 

subterranean  phenomena  together  render  it,  to  my  mind,  altogether  proba- 
ble that  the  fissure  is  continuous,  as  shown  upon  the  surface  map. 

The  evidence  of  the  structure  of  the  country  on  this  section  is,  for  the 
most  part,  far  less  detailed  than  that  obtained  for  some  of  the  others;  but 
it  is  sufficient  to  justify  considerable  confidence  in  the  general  features 
shown.  The  Forman  shaft  leaves  nothing  to  be  desired,  thanks  to  the  thor- 
oughly scientific  spirit  in  which  the  management  has  preserved  accurately 
labeled  specimens  from  all  levels,  as  well  as  temperature  observations.  A 
very  important  point  proved  by  the  shaft  is  that  the  diabase  does  not  extend 
so  far  south  as  this  line,  for  had  it  done  so  it  must  have  been  encountered 
between  the  hornblende-andesite  and  the  quartz-porphyry.  The  Caledonia 
works  were  also  open  to  inspection,  and  were  carefully  examined.  The 
three  other  shafts  were  closed,  but  the  information  afforded  by  the  dumps, 
in  connection  with  the  maps  of  the  workings  and  the  statements  of  employes 
as  to  the  drifts  from  which  the  different  divisions  of  the  dump-piles  came, 
and  correlated  with  the  data  obtained  on  the  surface  and  in  the  mines  stiU 
open,  gave  ample  evidence  as  to  the  order  of  occurrence  of  the  rocks. 

Diabase  nowhere  appears  on  this  section,  but  is  found  overlying  quartz- 
porphyry  at  the  Overman,  a  short  distance  to  the  north,  and  a  small  partial 
section  is  given  to  illustrate  this  occurrence. 

Cross-section  through  the  Union  shaft. To  the    UOrth  of   tho  main  LoDE,   aS  tO  the 

south  of  it,  the  evidences  of  dynamical  and  of  chemical  action  grow  sHghter, 
though  much  less  rapidly.  From  the  section  through  the  Union  shaft,  for 
example,  it  appears  that  on  the  main  northerly  branch  no  secondary  fissure 
has  formed,  and  since  the  Lode  is  here  divided  at  the  surface  into  at  least 
three  stringers,  a  sufficient  intensity  in  the  faulting  action  to  produce  a  well- 
marked  secondary  fissure  could  scarcely  be  anticipated.  The  south  branch 
of  Seven-Mile  Canon  has  cut  deeply  into  the  surface  here  presented.  If  the 
eroded  ground  were  restored  some  traces  of  a  logarithmic  surface  would  be 
visible.  The  lower  workings  from  the  Union  shaft  are  entirely  accessible, 
and  prove  that  the  diabase  contact  is  not  on  the  fissure  which  has  been 
chiefly  explored  to  the  north  of  this  plane,  but  diverges  from  the  strike  of 
the  main  Lode  towards  the  northeast.  A  line  of  heavy  croppings  exists  in 
this  general  direction,  and  probably  marks  the  contact.     A  comparison  of 


280  GEOLOGY  OF  THE  COMSTOCK  LODE. 

this  section  with  the  surface  map  and  with  the  horizontal  section  on  the 
Sutro  Tunnel  level  shows  that  the  contact  between  the  diabase  and  the  diorite 
being  steeper  than  the  dip  of  the  northern  branch  of  the  Lode,  the  fork  of 
the  vein  is  met  much  farther  north  on  the  lower  levels  than  at  the  surface. 
The  disturbing  influence  of  the  sharp  bend  in  the  diabase-diorite  contact 
upon  the  regularity  of  the  faulting  action  is  visible  in  the  larger  amount  of 
crushed  rock,  and  the  apparently  displaced  diorite  masses  on  this  section. 
Most  of  the  diorite  east  of  the  northerly  fissure  and  nearly  all  of  that  on 
the  lower  levels  is  porphyritic.  A  small  ore  body  occurred  near  the  crop- 
pings  on  the  northerly  branch.  Mr.  King  describes  the  ore  there  found  as 
"fragmentary  masses  of  blocky  quartz,  impregnated  with  native  gold, 
closely  resembling  the  California  auriferous  quartz."  The  little  ore  bodies 
on  the  2300  and  2400-foot  levels  are  more  like  the  ordinary  Comstock 
ores.  The  evidences  of  solfataric  action  are  very  strong  on  the  lower  levels 
of  this  section;  indeed,  the  decomposition  is  so  profound  as  to  make  litho- 
logical  determinations  a  matter  of  the  utmost  difficulty. 

Cross-section  through  the  Sierra  Nevada. The    Sierra    NcVUda     SeCtioU    shoWS     Cvi- 

dences  of  very  powerful  dynamical  action,  yet  of  but  a  small  amount  of 
faulting;  for  the  dip  of  the  north  fissure  is  here  so  irregular  that  no  move- 
ment whatever  could  occur  in  the  ordinary  direction  without  extensive  frac- 
turino".  The  occurrence  of  limestone  on  this  section  has  already  been 
noticed.  The  diorite  beneath  it  is  mainly  granular,  and  that  resting  upon  it 
is  for  the  most  part  porphyritic,  though  no  sharp  line  can  be  drawn  for  any 
considerable  distance  between  these  varieties.  It  appears  to  me  that  this  in- 
cluded sheet  of  stratified  rock  was  largely  instrumental,  by  its  weakening 
effect,  in  determining  the  course  of  the  north  fissure.  Beneath  the  lime- 
stone is  a  small  stringer  of  diabase,  no  doubt  connected  somewhere  with 
the  main  body  to  the  east,  but  at  what  point  is  uncertain.  It  is  accompa- 
nied by  a  minute  quantity  of  ore,  not  unlike  that  of  the  Comstock  bonanzas, 
but  it  would  be  difficult  to  gather  five  pounds  of  it,  and  there  is  no  likeli- 
hood of  any  ore  body  of  importance  being  found  here.  The  same  stringer 
of  diabase,  or  a  similar  one,  occurs  further  north  in  Utah  ground,  on  the 
north  fissure.  The  main  body  of  diabase  seems  to  have  been  struck  on  the 
1450  level  of  the  Sierra  Nevada  by  a  drill  hole,  the  cores  of  which  were 


THE  LODE.  281 

fortunately  preserved.     The  drift  itself  was  inaccessible,  and  could  not  have 
been  opened  at  any  moderate  cost. 

The  east-and-west  fault. — There  are  clear  evidences  of  a  slight  downward 
movement  to  the  north  of  the  Sierra  Nevada,  or  an  equivalent  rise  of  the 
region  to  the  south.  It  is  impossible  to  state  definitely  that  this  was  not 
independent  of  the  great  fault,  but  after  considerable  study  of  the  case  it  has 
seemed  to  me  unlikely,  on  the  whole,  that  the  two  movements  were  uncon- 
nected. Everything  shows  that  the  enaptive  rocks  of  the  District  are  exceed- 
ingly rigid,  and  canaot  be  flexed  perceptibly  without  breaking.  At  the 
same  time  there  is,  as  has  been  seen,  strong  proof  that  the  faulting  dimin- 
ishes rapidly  to  the  north  and  south,  beyond  the  points  at  which  the  main  Lode 
ramifies.  In  part  the  strain  was  weakened  by  distribution  over  vai-ious 
fissures,  but  this  would  have  been  insufficient  to  efffect  adjustment  in  the 
absence  of  flexibility.  This  argument  would  therefore  point  to  the  proba- 
bility of  east-and-west  fractures  as  a  means  of  relief,  and  it  is  to  this  action 
that  the  little  slips  in  the  Sierra  Nevada  appear  to  me  attributable. 

Cross-section  through  the  Utah. — In  the  Utttk  the  north  fissure  again  straightens, 
so  as  to  exhibit  approximately  the  usual  dip  of  the  Comstock,  and  though 
the  fault  was  slight  it  left  a  trace  of  a  secondary  fracture.  Diabase  appears 
in  several  levels,  but  only  as  an  irregular  dike,  backed  by  micaceous 
diorite,  which  also  shows  extensively  on  the  surface  in  this  neighborhood. 
As  nearly  as  can  be  made  out,  this  diabase  comes  in  on  a  cross-fissure  from 
the  southeast  and  not  on  the  branch  of  the  Lode.  The  evidences  of  solfa- 
taric  action  are  not  great  in  this  mine,  much  of  the  rock  being  even  fresher 
than  that  to  be  found  on  the  surface  at  any  point  in  the  District.  In  the 
lowest  levels,  however,  there  are  belts  of  somewhat  decomposed  rock. 

Horizontal  section. — It  was  intended  to  make  horizontal  sections  of  the  Com- 
stock on  three  levels,  but  this  proved  wholly  impracticable  on  account  of 
the  inaccessibility  of  the  older  workings.  Fortunately  it  was  possible  to 
explore  nearly  all  of  the  Sutro  Tunnel  level,  1,900  feet  below  the  croppings. 
The  result  is  recorded  in  Atlas-sheets  VIII.  and  IX.,  where  the  inaccessible 
drifts  are  shown  in  hair  lines;  while  the  projection  of  the  principal  workings 
on  other  levels,  of  which  use  was  made  in  drawing  inferences  as  to  the 
conditions  existing  on  the  1900-foot  level,  is  shown  in  dotted  lines.     The 


282  GEOLOGY  OF  THE  COMSTOCK  LODE. 

care  with  which  the  determinations  were  made  is  shown  by  the  abundance 
of  the  marks  indicating  the  points  from  which  specimens  were  collected, 
and  slides  ground.  This  very  laborious  collection  was  necessitated  chiefly 
by  the  extreme  state  of  decomposition  of  the  rocks,  which  here  almost 
wholly  effaces  their  distinguishing  characteristics.  It  was  also  necessary  to 
prove  the  presence  or  absence  of  any  rock  which  could  properly  be  brought 
under  the  definition  of  propylite. 

Ore  bodies  occur  at  the  diabase  contact. — It  appears  from  this  scctiou  that  the  east 
wall  of  the  Comstock,  from  the  Overman  to  the  Sierra  Nevada,  is  diabase, 
while  the  west  wall  is  diorite  for  only  a  part  of  the  distance.  By  comparison 
with  the  vertical  sections  and  the  vertical  projection  of  the  Lode  it  will 
be  seen  that  all  the  ore  bodies  of  any  importance,  except  that  in  the  Justice, 
are  at  or  close  to  the  diabase;  while  the  Gold  Hill  bonanzas  rest  upon  met- 
amorphic  rocks.  The  forking  of  the  vein  at  the  Overman  is  well  exhibited 
on  this  level,  with  its  cause,  the  divergence  of  the  black  dike  from  the  main 
diabase  mass.  To  the  north  it  is  evident  that  the  north  fissure  is  on  the 
strike  of  the  Lode,  and  that  its  formation  was  probably  facilitated  by  the 
presence  of  the  hmestone  body  in  the  Sierra  Nevada  ground. 

Faulting. — The  evidence  with  regard  to  faulting  offered  by  this  level  is 
interesting.  The  course  of  the  Lode  is  very  closely  the  same  as  the  line  of 
the  croppings,  with  the  exception  of  the  undulation  shown  at  the  surface 
opposite  the  Virginia  group  of  bonanzas.  The  disappearance  of  this  undu- 
lation was  discussed  in  connection  with  the  vertical  section  through  the 
Sutro  Tunnel.  The  effect  of  the  compression  produced  by  the  sharp  bend 
of  the  diabase  contact  to  the  eastward  at  the  north  end,  in  conjunction  with 
the  southeasterly  dip,  is  seen  in  the  great  mass  of  crushed  rock  in  the 
northern  mines.  This  crushed  rock  has  been  denominated  vein  matter,  in 
accordance  with  local  mining  usage,  because  it  is  decomposed  past  certain 
lithological  determination;  it  is  not  laid  down  as  forming  a  part  of  the 
vein,  however,  because  it  is  not  a  loose  aggregation  of  fragments  considerably 
removed  from  their  original  position,  but  consists  of  huge  rock  masses  fis- 
sured in  every  direction. 

Close  contact  of  the  walls. — Considering  the  extent  of  the  vein  and  the  indu- 
bitable evidences  of  an  extensive  fault,  it  is  at  first  sight  very  remarkable 


THE  LODE.  283 

that  the  walls  are  almost  everywhere  in  such  close  contact,  and  that  the 
only  large  opening  due  to  mere  relative  displacement  of  the  walls  is  that 
occupied  by  the  Gold  Hill  bonanza.  If  the  theory  of  the  fault  propounded 
in  Chapter  IV.  is  correct,  howe<^er,  this  state  of  things  follows  as  a  necessary 
consequence,  for  the  vein  represents  only  a  single  parting,  and  the  relative 
motion  between  its  walls  is  the  relative  motion  of  two  successive  sheets. 
The  actual  amount  of  displacement  must  depend  on  the  thickness  of  the 
sheets,  which  on  the  Comstock  is  certainly  not  above  twenty-five  feet.  This 
would  answer  to  a  relative  movement  of  the  actual  walls  of  something  like  a 
hundred  feet.  The  opening  of  the  vein  in  Gold  Hill  is  probably  in  part 
attributable  to  the  character  of  the  foot  wall,  which,  being  stratified  at  an 
angle  to  the  Lode,  would  be,  as  all  experience  shows,  less  rigid  and  less 
easily  spHt  into  sheets.  The  dip  of  the  west  wall  in  Gold  Hill  is  also  con- 
siderably smaller  than  in  Virginia,  about  10°  less,  and  this  fact  must  have 
had  a  tendency  to  ease  the  pressure  in  the  southern  mines.     • 

Influence  on  the  path  of  rising  waters. — Oh  account  of  thc  Small  relative  movemeut 
of  the  walls  of  the  Lode  these  are  sometimes  found  nearly  or  quite  in  con- 
tact with  one  another  over  considerable  areas;  and  at  points  where  the  walls 
are  perceptibly,  but  not  distantly,  separated  the  intervening  space  is  often 
closely  packed  with  clay  and  rock  fragments.  The  vein  is  therefore  not  an 
open  water  channel  throughout,  and  it  is  highly  probable  that  on  some 
straight  or  sinuous  line  it  may  be  impenetrable  to  liquids  from  one  end  to 
the  other.  With  the  east  country  rock  the  case  is  different.  As  has  been 
noticed  frequently  in  the  foregoing  pages,  it  shows  an  endless  number  of 
partings  parallel  to  the  Lode  and  innumerable  fractures  across  the  sheets. 
Few  of  these  partings  show  any  clay,  and  as  capillary  fissures  can  never  be 
stopped  except  by  plastic  material,  there  is  httle  obstruction  to  the  circula- 
tion of  water  in  the  country  rock.  This  condition  of  things  has  most  likely 
had  not  a  little  to  do  with  the  deposition  of  ore.  The  waters,  rising  from  a 
depth  which  the  heat  relations  show  must  be  measured  in  miles,  were  pre- 
vented from  following  the  Lode  fissure  and  were  forced  to  permeate  the  coun- 
try rock,  reaching  the  open  spaces  of  the  vein  laterally,  and  there  depositing 
the  quartz  and  ore  minerals  dissolved. 

Partial  section  on  the  asoo-foot  level. — The  uorthem  miues  wcrc  accessible  on  the 


284  GEOLOGY  OF  THE  COMSTOCK  LODE. 

250U-foot  level  for  a  considerable  distance,  and  a  horizontal  section  of 
these  workings  is  presented.  It  shows,  in  connection  with  the  parallel 
section  600  feet  above,  the  growing  tendency  of  the  diabase  contact  lo  dip 
towards  the  southeast  and  the  great  increase  of  crushed  rock  with  increasing 
depth.  All  preparations  had  been  made  to  lay  down  the  geology  of  the 
Gold  Hill  mines  at  the  corresponding  level,  when  a  flood  rendered  the 
workings  inaccessible.  The  map,  however,  at  least  indicates  the  continuity 
of  the  vein  in  depth  and  parallelism  of  structure  between  this  and  the  Sutro 
Tunnel  levels. 

Vertical  projection  of  bonanzas. — Thc  lougitudinal  vcrtical  projection  needs  no 
explanation,  supplementing  in  an  evident  manner  the  other  Atlas-sheets. 
The  disposition  of  the  various  bonanzas  which  it  shows  has  been  mentioned 
in  connection  with  the  cross-sections  of  the  Lode.^ 

Mine  maps. — Tho  cutirc  official  mine  maps  are  also  presented,  and  will 
enable  those  specially  interested  in  the  Lode  to  follow  out  many  details  of 
structure.  The  notes  on  these  maps  as  to  walls,  clay  seams,  etc.,  represent 
the  deliberate  judgment  of  the  surveyors  and  superintendents,  and  I  have 
found  them,  where  accessible,  for  the  most  part,  correct.  They  are  left  as 
they  stood  on  the  originals,  because  the  greater  number  of  the  locahties 
where  they  occur  are  inaccessible,  and  as  a  record  of  opinion  of  those 
technically  engaged  in  mining  they  have  a  distinct  value,  which  would  be 
lost  if  partially  replaced  by  my  own  determinations.  Not  all  the  galleries 
appear  on  the  maps,  for,  though  the  main  workings  have  been  carefully 
plotted  from  the  earliest  times,  unimportant  drifts  are  often  run  without  the 
cooperation  of  the  surveyor,  and  these  sometimes  escape  record.  The  sur- 
veyed galleries,  shafts,  and  winzes  aggregate  about  165  miles,  and  the  un- 
recorded ones  probably  30  miles  additional.* 

Claim-map. — Thc  claim-map  of  the  Washoe  District  forms  a  proper  com- 
plement to  the  mine  maps.    It  shows  the  claims  up  to  1 881  and  distinguishes 

'In  preparing  all  of  the  geological  sections  of  the  Lode,  I  was  assisted  by  Mr.  K.  H.  Stretch, 
■who  is  responsible  only  for  the  mapping,  the  geological  determinations  being  my  own.  My  determina- 
tions, however,  were  greatly  faciUtated  by  Mr.  Stretch's  familiarity  with  the  old  workings,  now  for  tha 
most  part  inaccessible,  and  by  his  zealous  assistance  in  gathering  data  as  to  structure  and  lithology. 
The  longitudinal  vertical  projection  of  the  Lode  is  entirely  Mr.  Stretch's  work. 

i'The  official  surveyors  of  the  Comstock  have  been  Messrs.  I.  E.  James,  E.  H.  Stretch,  Marietta 
&  Hunt,  T.  D.  Parkinson,  Browne,  Hoffmann  &  Craven,  and  L.  F.  J.  Wrinkle.  The  contract  for  the 
maps  was  made  with  Messrs.  Hoffmann  &  Craven. 


THE  LODE.  285 

claims  for  which  patents  have  been  issued,  those  on  which  applications  for  a 
patent  have  been  made,  those  determined  by  U.  S.  survey,  but  on  which  no 
applications  for  patents  have  been  made,  and  finally  claims  the  boundaries 
of  which  have  merely  been  determined  by  private  survey.  An  index  to 
the  claims,  showing  the  position  of  each  on  the  map,  will  be  found  at  the 
end  of  the  volume.^ 

Conclusions, — Collcctively,  the  various  observations  made,  if  they  are  correct 
and  the  inferences  from  them  sound,  throw  considerable  light  on  the  history 
of  the  Lode.  After  the  eruption  of  the  diorite  the  first  event  of  importance, 
so  far  as  the  vein  is  concerned,  was  the  outburst  of  diabase,  which  involved 
a  rupture  and  dislocation  of  the  earlier  diorite,  leaving  a  smooth  contact 
between  the  two  rocks  at  an  angle  of  about  45°.  The  contact  was  after- 
wards slightly  opened  to  admit  the  younger  diabase  or  black  dike.  Erup- 
tions of  earlier  hornblende-andesite  and  of  augite-andesite  afterwards 
occurred,  which  probably  caused  fractures  and  dislocation  in  the  eastern 
portion  of  the  diabase,  but  produced  no  traceable  action  on  the  Comstock 
fissure.  The  country  was  subsequently  so  eroded  as  to  reduce  the  surface 
of  these  four  rocks  to  a  gently  sloping  plain,  with  an  inclination  of  a  little 
more  than  two  degrees  to  the  west.  After  the  commencement  of  the  dry 
period  (dry,  that  is  to  •  say,  so  far  as  this  region  was  concerned)  a  great 
movement  began  which  may  possibly  have  been  a  sinking  of  the  hanging 
wall,  but  was  more  probably  a  rise  of  the  foot  wall.  The  center  of  action 
appears  to  have  been  near  Mount  Davidson.  This  dislocation  involved  an 
enormous  friction,  one  result  of  which  was  a  separation  of  the  foot  wall  and 
the  hanging  wall  into  sheets  parallel  to  the  fissure  for  a  long  distance  from 
it.  A  secondary  effect  of  the  same  force  was  the  formation  of  inimmerable 
cracks  in  these  sheets  nearly  perpendicular  to  their  partings.  The  edge  of 
the  east  country  necessarily  assumed  the  form  of  a  wedge,  and  was  broken 
completely  through  at  a  point  a  few  hundred  feet  below  that  at  which  the 
primary  fissure  reached  the  sui-face.  Openings  were  formed  along  the  Com- 
stock as  a  result  of  the  movement  of  the  walls,  but  under  a  variety  of 
circumstances.  In  Gold  Hill  a  space  was  left  by  the  non-conformity  of  the 
wall  surfaces  brought  into    opposition.     In  the  Virginia  group  a  slight 

'  The  claim-map  was  prepared  by  Messrs.  HoflFmann  &  Craven.  Some  additions  and  corrections, 
however,  were  made  by  Mr.  Wrinkle. 


286  GEOLOGY  OF  THE  COMSTOCK  LODE. 

irregularity  in  the  dip  of  the  foot  wall  prevented  the  mass  broken  from  the 
edge  of  the  east  country  from  following  the  main  body  of  diabase  to  its 
final  position ;  while  in  the  Consolidated  Virginia  and  the  neighboring  mines, 
at  a  depth  of  between  1,000  and  2,000  feet,  a  projecting  mass  upon  the  foot 
wall  gave  rise  to  a  local  rent  in  the  hanging  wall.  Besides  these  more 
iinportant  openings,  numerous  clefts  formed  in  the  prismatic  horse  which 
had  been  broken  off  from  the  hanging  wall,  and  between  the  horse  and 
the  main  body  of  the  east  country.  Large  quantities  of  rock  were  ground 
to  dust  in  the  course  of  the  faulting,  especially  at  and  near  the  great  horse, 
where  the  mechanical  action  was  least  regular. 

Floods  of  heated  waters  now  rose  from  a  depth  of  two  or  more  miles, 
certainly  carrying  carbonic  and  sulphhydric  acids,  and  possibly  other  active 
reagents,  in  solution.  The  water  followed  the  course  of  the  main  fissure  as 
closely  as  circumstances  permitted,  but  was  deflected  to  a  great  extent  into 
the  fractured  mass  of  the  east  country,  where  decomposition  resulted,  ^ihca 
and  metallic  salts  were  set  free  from  the  mineral  constituents  of  the  rock, 
and  were  carried  into  the  comparatively  open  spaces  near  the  main  fissure, 
where  they  were  redeposited.  The  proportion  of  silica  to  ore  minerals 
varied  greatly  with  time  and  local  circumstances,  which  if  Xhej  are  capable 
of  full  explanation  certainly  have  not  received  it  in  this  report.  Some  of  the 
causes  of  the  variations,  however,  can  be  indicated  without  difficulty.  The 
lithological  character  of  the  rock  upon  which  the  waters  acted  was  evidently 
of  prime  impoi'tance,  determining  both  metallic  contents  and  gangue;  so  that 
the  deposits  of  Cedar  Hill,  those  of  the  Justice  mine,  and  the  bonanzas  of  the 
main  Lode,  all  show  distinctive  characters.  The  duration  of  the  exposure 
of  particular  rock  masses  to  solvent  action  no  doubt  had  much  to  do  with 
the  tenor  of  the  resulting  ore.  It  is  hkely,  for  example,  that  sihca  under  the 
conditions  then  prevailing,  is  more  readily  soluble  than  silver  compounds. 
If  so,  the  water  first  passing  over  a  mass  of  rock  would  deposit  low-grade 
quartz  in  the  vein,  and  subsequently,  as  the  supply  of  soluble  silica  dimin- 
ished, a  better  quahty.  It  seems  clear  that  fresh  movements  occurred  from 
time  to  time,  and  that  fresh  rock  surfaces  were  thus  exposed.  This  would 
have  brought  about  alternations  in  richness,  such  as  have  sometimes  been 
noticed  in  the  Lode.     Pressure,  too,  if  not  temperature,  may  have  varied 


THE  LODE.  287 

from  time  to  time,  and  so  may  the  quantity  of  active  reagents  in  the  rising 
waters.  On  the  whole,  the  earlier  deposits  of  quartz  seem  to  have  been  of 
lower  grade  than  the  later  ones,  but  the  phenomena  are  so  complicated  that 
no  considerable  practical  value  attaches  to  this  observation. 

The  ore  was  deposited  on  the  walls  and  fragments  of  rock  as  in  more 
regular  veins,  but  the  currents  percolating  from  the  east  and  decomposing 
the  rock  through  which  they  passed,  gave  the  east  wall  a  somewhat  indefi- 
nite character.  This  indefiniteness  was  increased  by  the  dynamical  action 
which  followed  the  deposition  of  quartz,  and  probably  also  accompanied  it. 
After  most  of  the  quartz  was  precipitated,  renewed  movements  occurred, 
crushing  the  deposits  in  great  part  to  so-called  "  sugar  quartz."  It  was  the 
quartz  bodies  standing  at  a  considerable  angle  to  the  west  wall,  and  there- 
fore crossing  the  fissure  planes,  which  were  most  extensively  comminuted. 
More  attrition  products  were  of  course  also  formed  at  the  same  time. 

The  solutions  which  so  powerfully  attacked  the  polyhedral  fragments 
of  diabase  were  of  course  not  without  efi"ect  on  the  pulverized  rock  masses 
which  were  abundant,  particularly  in  and  near  the  secondary  fracture, 
or  "  east  vein."  The  clays  are  the  result.  In  a  simple  vein,  attrition  mix- 
tures and  clays  are  apt  to  occur  only  on  the  two  walls.  On  the  Comstock 
such  a  regular  formation  is  found  on  the  west  wall,  but  seldom  on  the  east. 
There  is  no  necessary  connection  between  walls  and  clays  in  spite  of  their 
frequent  association,  some  typical  veins  showing  nothing  of  the  kind.  The 
clays  of  the  Comstock  show  little  kaolin. 

Probabilities. — Thc  first  couditiou  for  a  deposit  of  ore  is  the  formation  of 
an  opening,  and  on  the  Comstock  such  spaces  appear  to  have  formed  in 
three  distinct  ways,  already  explained.  The  secondary  fracture  has  been 
worked  out,  and  except  in  Gold  Hill  considerable  nonconformity  of  the  walls 
is  not  to  be  looked  for.  There  it  is  as  likely  to  occur  at  greater  depths  as 
above  ;  indeed,  the  fact  of  its  occurrence  in  the  Crown  Point  and  Belcher,  at 
a  mean  depth  of,  say,  1,700  feet  from  the  Gould  d  Curry  croppings,  leads 
almost  necessarily  to  the  conclusion  that  there  must  be  other  nonconformi- 
ties at  greater  depths,  unless  the  rocks  change  to  other  species.  Openings 
of  the  type  of  that  which  contained  the  Consolidated  Virginia  and  California 
bonanza  may  occur  at  any  point  on  the  vein,  and  wholly  without  warning 


288  GEOLOGY  OF  THE  COMSTOCK  LODE. 

from  above,  as  was  the  case  with  that  body.  The  want  of  indications  of 
such  an  opening  from  above  is  due  simply  to  the  fact  that  from  the  nature 
of  the  case  the  accompanying  subsidiary  phenomena  are  on  lower  levels  than 
the  opening.  At  least  one  other  type  of  opening  may  occur,  which  is  as 
likely  to  carry  ore  as  those  just  mentioned.  Where  large  bodies  of  rock 
are  broken  and  dislocated,  interstitial  spaces  of  considerable  size  may  readily 
form  within  the  mass.  An  enormous  volume  of  such  material  exists  in  the 
lower  levels  of  the  north  end  mines,  and  nothing  would  be  less  surprising 
than  the  discovery  of  one  or  more  ore  bodies  in  that  locality.  Attendant 
upon  the  ore  bodies  and  somewhat  below  them  to  the  east,  the  hanging  wall 
will  probably  be  more  heavily  charged  with  pyrite  than  the  average  rock 
of  the  east  country,  as  has  been  the  case  with  former  bonanzas. 

Of  the  actual  precipitation  of  ore  and  gangue  from  solution  little  is 
known.  It  is  very  natural  to  connect  it  with  surface  influences,  and  hence 
to  suppose  that  ore  must  be  limited  to  certain  depths.  Such  an  hypothesis 
is  frequently  held  by  mining  men,  but  experience  does  not  confirm  it ;  for 
though  there  are  shallow  deposits,  there  are  many  deep  ones.  The  gold 
veins  of  California  and  Australia  show  no  tendency  to  give  out  in  depth, 
when  affected  by  no  other  unfavorable  conditions,  such  as  a  change  of  rock; 
and  the  mines  of  Pribram,  in  Bohemia  (the  only  ones,  I  believe,  which  are 
deeper  than  those  on  the  Comstock),  were  never  so  rich  and  profitable  as 
they  have  been  since  the  3,000-foot  level  was  passed. 

The  western  limit  of  the  diabase  is  the  only  ground  in  which  impor- 
tant ore  bodies  ever  have  been  or  are  ever  likely  to  be  found  in  the  Com- 
STonK  mines,  and  exploration  should,  in  my  judgment,  be  confined  to  the 
neighborhood  of  this  contact.  Money  spent  elsewhere  will  almost  certainly 
be  wasted.  As  long  as  the  east  country  continues  to  show  an  extensive  body 
of  diabase,  there  is  no  i-eason  for  discouragement.  Should  this  rock  ever  nar- 
row to  a  mere  dike  between  diorite  walls,  the  outlook  would  be  gloomy; 
but  it  is  highly  probable  that  such  a  change  occurs,  if  at  all,  only  at  a 
point  far  below  the  limit  which  technical  difficulties  will  set  to  exploration. 

The  whole  contact  between  diabase  and  the  underlying  rocks  is  worthy 
of  careful  exploration.  Evidences  of  disturbance  and  decomposition  are  to 
be  regarded  as  indications  of  the  possible  neighborhood  of  ore,  and  regions 


THE  LODE.  289 

exhibiting  these  characteristics  should  be  thoroughly  cross-cut ;  while,  where 
the  rock  is  comparatively  firm  and  fresh,  drifts  or  winzes  should  be  pushed 
on  to  more  promising  ground.  The  country  northeast  of  the  Ophir  is  par- 
ticularly favorable.  As  may  be  seen  from  the  horizontal  sections,  it  pre- 
sents a  large  extent  of  unprospected  contact  between  diabase  and  diorite 
directly  adjoining  a  region  of  broken  and  highly  altered  rock  where  ore 
in  small  quantities  has  already  been  found.  Ore  is  not  unlikely  to  be  met 
with  in  this  unexplored  area  at  depths  of  less  than  2,000  feet,  and  therefore 
under  comparatively  favorable  conditions  as  to  heat  and  water.  The  mines 
near  the  Union  shaft  are  also  likely  to  find  ore  towards  the  bottom  of  the 
mass  of  shattered  rock  in  which  the  1,900  and  2,.500-foot  levels  are  exca- 
vated. In  the  Best  <&  Belcher  ground,  too,  there  are  signs  of  great  disturb- 
ance, though  the  decomposition  is  less  intense  than  in  the  mines  north  of  the 
Ophir.  A  drift  from  the  lowest  levels  of  the  Consolidated  Virginia  would 
show  whether  the  indications  on  this  claim  improve  with  depth. 


CHAPTER    IX. 

ON  THE  THERMAL  EFFECT  OF  THE  ACTION  OF  AQUEOUS 
VAPOR  ON  FELDSPATHIC  ROCKS. 

BY  CARL  BAKT7S. 

Mr.  Church/  in  his  report  on  the  geology  of  the  Comstock  Lode,  has  en- 
deavored to  account  for  the  abnormally  rapid  increase  of  the  temperature 
of  this  District  with  increasing  depth^  by  ascribing  it  to  chemical  action — 
more  immediately  to  the  decomposition  (kaolinization)  of  the  feldspathic 
rocks  in  consequence  of  the  presence  of  moisture.  This  theory,  however, 
notwithstanding  the  ingenuity  with  which  it  has  been  discussed  by  its 
author,  is  based  on  an  assumption  that  has  scarcely  a  single  experimental 
datum  to  support  it;  nor  is  the  fundamental  hypothesis  upon  which  Mr. 
Church  bases  his  argument,  namely,  that  the  process  of  kaolinization  is  one 
from  which  we  may,  a  priori,  expect  the  production  of  heat  (as  Mr.  Becker 
has  already  pointed  out)  by  any  means  of  a  kind  to  be  readily  admitted. 

General  plan. — It  appeared  Very  desirable,  therefore,  insomuch  as  from 
theoretical  grounds  alone  there  is  abundant  room  for  difference  of  opinion, 
to  put  the  matter  to  a  direct  physical  test.  At  the  outstart,  and  with  the 
time  and  means  available  in  camp,  qualitative  experimentation  only  could 
judiciously  be  attempted,  the  necessarily  complicated  quantitative  study 
being  reserved  for  more  favorable  opportunities ;  if,  indeed,  the  preliminary 
investigation  should  furnish  results  of  sufficient  interest  to  warrant  further 
research. 

The  thermal  effect  of  kaolinization  (abbreviated  T.  E.  K.)  may  be  de- 
fined as  the  quantity  of  heat  produced  by  the  action  of  aqueous  vapor  on 


290 


'  The  Comstock  Lode,  its  formation  and  history,  by  John  A.  Church,  1879. 
'  This  volume,  Chapter  VII. 


EXPEEIMENTS  ON  KAOLINIZATION.  291 

the  unit  mass  of  feldspathic  rock  in  the  unit  of  time.  T.  E.  K.  may,  there- 
fore, a  priori,  be  either  positive,  zero,  or  negative.  It  must  be  regarded, 
moreover,  as  a  function  of  the  time  during  which  the  action  has  been  going 
on,  of  the  temperature  and  of  the  quantity  of  feldspar  contained  in  a 
given  sample  of  rock. 

The  problem  presented  is  none  other  than  the  measurement  of  very 
small  increments  of  temperature  with  all  the  accuracy  attainable.    For  such 
a  purpose  either  thermometric  or  electrical  means  are  applicable.     The  for- 
mer requiring  specially  constructed  apparatus,  had  at  once  to  be  discarded. 
It  IS  a  question,  moreover,  whether  the  thermometric  method  of  research 
will  not,  under  all  circumstances,  offer  obstacles  of  a  very  serious  character. 
In  the  measurement  of  small  increments  at  the  boihng  point  it  becomes  a 
matter  of  great  importance  to  keep  the  mercury  column  throughout  at  a 
temperature  as  nearly  as  possible  equal  to  that  of  the  bulb— a  condition 
which  can  be  reaHzed  only  with  great  difficulty,  when  a  division  of  the  stem 
into  very  small  fractions  of  a  degree  is  also  required.^     Electrically,  there 
are  two  methods  applicable      The  first,  however,  based  on  the  relation 
between  temperature  and  resistance,  would  have  necessitated  the  measure- 
ment of  increments  of  the  latter  quantity  amounting  to  scarcely  0.0005  per 
cent,  of  the  whole,  in  order  to  arrive  at  the  accuracy  desired.     Though 
even  this  is  feasible  in  the  laboratory,  I  despaired  of  being  able  to  reach 
such  nicety  with  the  means  at  my  disposal.     In  view  of  these  facts,  it  was 
finally  determined  to  try  how  far  a  thermo-electric  method  of  research 
might  be  successful  in  answering  the  question. 

Processes  of  this  kind,  in  which  the  effect  observed  is  due  to  chemical 
action,  are  usually  accelerated  by  the  application  of  heat.  In  other  words 
the  assumption  is  warranted  that  the  thermal  effect  of  the  action  of  aqueous 
vapor  on  feldspar  (T.  E.  K.)  will  increase,  and  will  therefore  be  more  easily 
detected  as  the  temperature  of  the  vapor  increases;  provided,  of  course,  that 
this  temperature  is  not  chosen  so  high  as  to  dissociate  the  products  of  de- 

'  A  greater  difficulty  still  \POuld  probably  be  encountered  from  the  fact  that  the  reservoir  of  a 
thermometer  subjected  to  large  differences  of  temperature  is  by  no  means  constant  in  volume,  but  sub- 
ject to  variations  dependent  upon  the  glass  chosen.     (Phenomena  of  "after-action.") 


292 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


composition  resulting  in  a  normal  case.  Believing,  therefore,  that  the  phe- 
nomena of  kaolinization  are  reproduced  at  all  temperatures  below  a  certain 
limit,  and  that  the  difference  in  effect  is  merely  quantitative,  the  rock  in 
the  experiments  here  described  was  subjected  to  steam  at  the  boiling  point 
of  water  on  the  Comstock.'  Besides  this,  it  was  intended  to  modify  the 
method  of  research  sufficiently  to  trace  the  action  of  superheated  steam 
also.     This  must,  however,  be  reserved  for  a  future  report. 


Fig.  19. — Boiler  (scale  one-tifth). 

Apparatus. — The  apparatus  (a  boiler)  in  which  the  rock  was  subjected  to  the 
action  of  steam  is  shown  in  longitudinal  section,  on  a  scale  of  one-fifth, 


'  About  93°  C. 


/ 


EXPERIMENTS  ON  KAOLINIZATION.  293 

in  Fig.  1 9.  As  will  be  seen,  the  well-known  contrivance  for  determining  the 
boiling  point  of  thermometers  was  made  the  pattern  of  construction.  Steam 
is  generated  in  the  interior  conical  compartment  ahecdoi  heavy  tinned  slieet 
iron,  1 8  inches  in  diameter  at  the  bottom  and  12  inches  at  the  top,  and  between 
18  and  20  inches  high.  The  top,  dee,  also  conical,^  and  provided  with  a 
hole  at  c  for  the  escape  of  steam,  can  be  removed,  and  fits  like  a  lid  over 
the  walls  of  this  compartment.  The  whole  is  surrounded  by  the  cylindrical 
mantle,  fg  i  hf,  of  the  same  material.  The  top,  g  ih,  of  this  can  also  be 
removed,  has  the  form  of  an  ordinary  lid,  and  is  provided  with  tubulures 
for  the  insertion  of  corks,  etc.,  at  h  and  i.  The  exterior  compartment  com- 
municates with  the  air  by  the  tubulures/ and y. 

In  the  interior  of  the  inner  compartment,  and  held  in  position  by  a 
suitable  tripod  (not  shown  in  the  cut),  is  the  cylindrical  chamber  r  s  u  t,\\ 
inches  in  diameter  and  12  inches  high,  and  provided — like  a  sieve — with  a 
bottom  of  wire  gauze  strengthened  by  radial  supports  of  thick  brass  wire. 
P  q,  finally,  is  a  feed  pipe  for  resupplying  the  boiler  with  water  lost  by 
evaporation. 

The  rock  to  be  tested  was 'broken  into  small  fragments,  from  the  size 
of  a  hazel-nut  down  to  that  of  a  pin-head,  but  excluding  dust,  and  placed 
in  the  chamber  r  s  u  t.  Previously,  however,  the  thermo-element  x  y  z 
(described  on  the  next  page)  had  been  fixed  in  position,  supported  by  suita- 
ble cross-bars  of  wood  covered  with  thick  sheet  rubber.  In  putting  the 
rock  into  the  chamber  care  was  taken  to  pack  it  sufficiently  tight  to  prevent 
currents  of  steam  from  possibly  passing  through  the  mass.  Steam  reached 
the  interior  by  a  process  of  difi"usion,  thoroughly  saturating  the  whole.  Of 
this  I  had  frequent  occasion  to  convince  myself  Water  having  been  poured 
into  the  boiler  to  a  level  /  /,  approximately,  and  heated  to  ebullition,  the 
steam  completely  enveloped  the  rock  chamber,  permeating  the  material  in 
its  interior.  Passing  through  the  hole  c,  and  again  around  the  greater  part 
of  the  apparatus,  it  finally  escaped  at  /  and  f  into  the  air. 

As  a  source  of  heat,  two  small  kerosene  stoves  were  found  excellent. 
By  means  of  the  four  broad  flames  thus  obtained,  the  heat  could  be  regu- 

'  Thus  serving  a  second  purpose,  namely,  to  prevent  steam  condensed  on  the  top  of  the  boiler 
from  dripping  into  the  rock  below. 


294  GEOLOGY  OF  THE  COMSTOCK  LODE. 

lated  as  desired  and  kept  constant  during  the  whole  time  of  experimentation. 
Oil  could  be  supplied  without  interfering  with  the  flames.  Trimming  of 
wicks  was  seldom  necessary,  and,  there  being  four  flames,  gave  rise  to  no 
serious  disturbance.  To  diminish  the  heat  lost  by  radiation  as  much  as 
possible,  the  whole  apparatus,  with  the  exception  of  the  bottom,  was  cov- 
ered to  a  thickness  of  about  three-quarters  of  an  inch  with  cotton  batting, 
wrapped  in  layers  and  surrounded  externally  by  heavy  paper.  Finally, 
the  water  lost  by  evaporation  was  replaced  drop  by  drop  by  means  of  a 
pneumatic  arrangement  placed  upon  the  boiler,  but  not  shown  in  the 
figure.  The  number  of  drops  fed  in  a  given  time  was  so  regulated  by 
the  aid  of  a  small  faucet  as  to  keep  the  level  I  I  of  the  water  in  the  boiler, 
as  indicated  by  the  gauge  m  n,  approximately  at  a  constant  height.  The 
ebullition  was  not  allowed  to  become  sufficiently  intense  to  produce  an 
increase  of  pressure  in  the  interior. 

To  recapitulate :  By  the  aid  of  a  fairly  constant  source  of  heat,  the 
ebullition  from  a  water  level  of  constant  height  could  be  maintained  at  a 
nearly  constant  intensity.  It  was  believed,  therefore,  that  a  stationary  ther- 
mal condition  would  soon  set  in  and  continue  indefinitely.  Errors  due  to 
fluctuation  of  the  barometric  column,  this  being  as  likely  to  produce  posi- 
tive as  negative  efi'ects,  could  be  excluded  by  proper  methods  of  reduction. 

Thermo-eisment. — To  measurc  thc  Small  increments  of  temperature,  a  thermo- 
pile composed  of  three  bismuth-silver  elements  was  first  used.  Though 
this  acted  well,  there  was  danger,  in  consequence  of  the  amount  of  sulphur 
in  the  rocks  (Fe  S^),  of  complete  destruction  of  the  silver  terminals  during  the 
course  of  the  experiment.  This  metal  was  therefore  discarded,  and  platinum, 
which  is  not  thus  afi"ected,  chosen  in  its  stead.  The  bismuth  was  cast  in  the 
shape  of  three  adjacent  sides  of  a  rectangle,  the  length  and  width  chosen  being 
such  as  to  allow  the  two  ends  to  occupy  the  positions  x  and  s  shown  in  Fig. 
19.  Of  course  care  was  taken  to  insulate  the  whole  thoroughly  from  the 
walls  of  the  boiler,  this  being  accompHshed  by  surrounding  the  element  on 
all  sides  by  strips  of  thick  sheet  rubber.  The  parts  of  the  element  were 
kept  from  touching  each  other  by  pieces  of  glass  tubing  suitably  placed. 
The  terminals — which,  to  prevent  confusion,  are  not  indicated  in  the  figure — 
were  themselves  insulated  by  a  covering  of  rubber  hose  of  small  caliber. 


EXPERIMENTS  ON  KAOLINIZATIOK. 


295 


They  passed  out  of  the  boiler  through  tubulures  (also  omitted  in  the  figure) 
placed  conveniently  on  its  sides,  the  hose  and  wire  being  secured  by  small 
perforated  corks.  Of  course  no  attention  was  paid  to  the  purity  of  the 
metal  employed.  The  silver  and  bismuth  were  fastened  together  by  melting 
a  Httle  globule  on  the  end  of  the  silver  wire,  and  then  applying  it,  while 
still  hot,  to  the  end  of  a  bismuth  bar.  The  soldering  thus  produced  was 
very  perfect.  The  platinum  and  bismuth  had,  however,  to  be  soldered 
together  by  ordinary  means. 

Method  of  measurement. — Thc  relatiou  betwecu  thc  electromotlvc  force  e,  due 
to  the  temperatures  T  and  t  (T>»^)  of  the  ends  of  the  thermo-element,  can 
be  expressed  with  the  aid  of  two  constants,  a  and  h,  thus : 

e:=z{T-t)  \^a  +  h{T+ty\. 

But  as  T— ^  in  this  case  is  a  very  small  quantity  (a  few  hundredths  of 
a  degree), 

where  T  is  the  temperature  of  ebullition  of  the  water  as  given  by  the  aid  of 
the  barometer.  Knowing,  there- 
fore, a,  h,  and  the  barometric 
height  we  are  able  to  find  Jt, 
or  the  diff"erence  of  temperature 
between  the  interior  and  the 
exterior  of  the  rock  chamber  (x 
and  z  in  Fig.  19). 

For  the  measurement  of  e 
a  "zero"  method  was  employed. 
In  Fig.  20  a  diagram  of  the  con- 
nections as  actually  made  is 
given,  for  the  purpose  of  calling 
attention  to  a  few  details  of  im- 
portance    in    measurements     of  Fig.  20. -Disposition  of  apparatus. 

this  kind.     The  platinum  terminals  of  the  thermo-element  e  are  soldered  to 
copper  circuit  wires  at  P,  the  points  of  junction  being  immersed  in  a  reservoir 


296  GEOLOGY  OF  THE  COMSTOCK  LODE. 

filled  with  petroleum.  Before  each  observation  this  liquid  was  stirred.  The 
copper  wires  pass  through  the  commutator  B,  and  thence  the  one  through 
a  double  key  K  to  the  point  h,  the  other  through  the  galvanometer  G^  to 
the  point  a,  thus  completing  the  first  branch.  The  smaller  resistance  r 
forms  the  second  branch,  also  terminating  at  the  points  a  and  h.  For  the 
convenient  insertion  of  this  resistance  a  number  of  small  holes  were  bored 
in  a  thick  piece  of  wood  and  filled  with  mercury.  The  points  a  and  h  are 
connected  with  the  extreme  holes  of  the  series  by  means  of  strips  of  thick 
copper  foil.  Finally,  the  terminals  of  a  zinc  sulphate  Daniell  E  pass 
through  the  commutator  A,  thence  the  one  through  the  key  K  and  directly 
to  fe,  the  other  through  a  large  rheostat  R  (from  1  to  10,000  ohms),  and  by 
a  thick  wire,  C,  to  a,  completing  the  third  branch. 
When  the  current  in  (?i  is  zero 

^—-^  Rj^r'  ^^'  ^^^^  simply,  :=iE  ^, 

where  e  is  the  electromotive  force  ate,  E  at  E  in  the  figure;  where,  further- 
more, B  is  the  resistance  at  B,  r  s.tr  in  the  figure,  and  where  r  is  negligible 
in  comparison  with  B. 

Having  thus  described  the  general  method,  it  will  be  pertinent  to  men- 
tion a  few  of  the  more  important  details.     By  means  of  K  two  circuits 

conveying  currents  due  to  E  and  e,  respectively, 
are  closed.  It  is,  however,  necessary  that  they 
should  be  so  closed  as  to  act  simultaneously  (dif- 
ferentially) on  the  galvanometer  Gi;  for  if  the  cur- 
rent due  to  the  electromotive  force  e  were  to  act  alone 
Fig.  21.— Section  of  key.  gerious  disturbances  might  be  the  result.  This  can 
be  accomplished  by  the  following  simple  contrivance  in  the  construction  of 
the  key.  Fig.  21  gives  a  section  through  the  line  of  mercury  cups  c  d,  Fig. 
20.  Pieces  of  thick  copper  wire,  bent  as  shown,  are  fastened  to  a  thin 
piece  of  board,  capable  of  revolving  partially  about  a  horizontal  axis  parallel 
to  the  line  c  d.  In  this  way  the  pieces  m  and  n  can  be  dipped  into  the  mer- 
cury cups  under  their  extremities  or  lifted  out  of  them  together.  The  board 
is,  moreover,  provided  with  a  spring  so  arranged  as  to  keep  m  and  n  out  of 
the  cups,  and  the  circuit  therefore  remains  open,  unless  closed  by  the  ob- 


n  n 


EXPBEIMENTS  ON  KAOLINIZATION.  297 

server.  The  cups  corresponding  to  w,  and  conveying  the  current  due  to  the 
Daniell  E,  are,  however,  filled  vpith  mercury  to  a  level  a  little  higher  than 
the  rest.  Hence,  under  all  circumstances  the  circuit  containing  E,  and  not 
passing  through  G^,  is  closed  first.  A  moment  after,  however,  that  contain- 
ing e  and  Gi  is  also  completed,  but  it  will  be  obvious  that  the  effect  from 
e  and  E,  if  the  directions  of  these  electromotive  forces  are  properly  chosen, 
will  act  differentially  on  G^,  as  was  desired. 

The  electromotive  force  e,  obtained  as  above,  is  never  wholly  due  to 
the  thermo-element  e  alone,  but  contains  also  a  disturbing  electromotive 
force,  e,  resulting  from  the  accidental  distribution  of  tempjerature  in  the 
connections.  For  a  short  period  of  time  (that  of  an  observation)  e  may  be 
considered  as  nearly  constant,  or  at  least  varying  linearly.  In  order  to  elim- 
inate the  latter,  very  largely  at  least,  Dr.  Strouhal  and  myself,  in  analogous 
experiments,  inserted  the  two  commutators  A  and  B.  In  a  series  of  corre- 
sponding positions  of  the  commutators,  alternately  opposite,  direct  meas- 
urement would  give 


+E        ' 


-   --E -^ 

+e+eO.+2a)_ 


a. 


+E         --^^ 

— e-f£(l-|-3a) 


E         =""' 


+e+e(l+ia)_ 
+E 


=za, 


where  a  is  a  constant.  If  now  an  odd  number  of  observations  be  made, 
and  if  Jfj  be  the  mean  of  the  odd  right-hand  members,  M2  the  mean  of  the 
even  right-hand  members, 

In  the  present  investigation,  the  electromotive  forces  measured  being 
exceedingly  small,  at  least  five  commutations  of  both  A  and  B  were  made 
for  each  value  of  M  cited. 


298  GEOLOGY  OF  THE  COMSTOGK  LODE. 

The  galvanometer  (?i  was  one  of  low  resistance,  consisting  of  a  few 
hundred  turns  of  wire  around  an  astatic  needle  on  silk  fiber.  The  instru- 
ment was  quite  delicate,  and  with  the  aid  of  proper  methods  of  interpolation 
would  easily  have  enabled  the  measurement  of  increments  as  small  as  a  few 
ten-thousandths  of  a  degree  centigrade.  Unfortunately,  the  silk  was  too  thick, 
and  the  zero  point  of  the  instrument,  as  a  consequence,  too  variable ;  while, 
on  the  other  hand,  the  strong  winds  of  the  region  and  the  frail  foundation  of 
the  house  itself  rendered  this  accuracy  unattainable,  and  we  were  obliged 
to  content  ourselves  with  measurements  accurate  to  a  few  thousandths  of  a 
degree.     Readings  were  made  with  a  mirror  and  scale. 

Thus  far  E  has  been  considered  constant.  As  this  is  not  the  case,  its 
variations  were  measured  by  the  aid  of  a  second  galvanometer,  G'g  (Fig.  20), 
made  by  Mr.  Grunow,  and  described  elsewhere.^  This  instrument  was 
placed  at  a  distance  from  the  boiler,  in  Ae  cellar,  where  the  atmospheric 
condition  was  tolerably  uniform,  and  for  convenience  provided  with  a  com- 
mutator of  its  own,  D.  It  will  easily  be  seen  that  by  breaking  the  circuit 
at  B  and  C  and  closing  K,  E  will  be  in  a  simple  circuit,  including  G^,  and 
that  its  value  may  be  measured  in  terms  of  i2,  which  is  also  included. 

The  value  of  the  constants  a  and  h  in  the  equation  on  page  295  was 
determined  by  putting  the  ends  of  the  thermo-element  e  in  adjoining  jars, 
containing  water  at  different  temperatures.^  Ten  or  more  observations  were 
usually  made,  from  which  a  and  h  were  calculated  by  the  method  of  least 
squares. 

I  cannot  but  consider  this  method  of  measuring  differences  of  tempera- 
ture as  theoretically  very  perfect.  First  of  all,  discrepancies  due  to  Peltier's 
phenomena  are  avoided,  while  the  constants  a  and  h  are  used  pi'ecisely  in 
the  same  way  in  which  they  were  obtained.  Moreover  methods  of  interpo- 
lation are  particularly  applicable ;  even  a  method  of  multiplication  might 
be  thus  employed.  There  can  be  no  doubt  that  under  more  favorable  cir- 
cumstances the  minimum  difference  of  temperature  measurable  with  cer- 
tainty would  be  much  smaller  than  I  have  been  compelled  to  consider  it. 

1  This  volume,  page  327. 

2  Not  having  a  reliable  barometer,  all  temperatures  are  expressed  in  terms  of  the  interval  be- 
tween zero  and  100°  C.  of  the  best  instrument  at  hand,  this  interval  being  arbitrarily  assumed  as  correct. 
On  this  assumption  the  stems  of  the  thermometers  used  were  calibrated. 


EXPERIMENTS  ON  KAOLINIZATION. 


299 


Material  experimented  upon. — The  Tock  selected  bj  Mr.  Becker  for  these  exper- 
iments was  the  freshest  diabase  encountered  in  the  mines.  The  feldspars 
show  scarcely  a  trace  of  decomposition,  and  a  large  part  of  the  augite  is 
unaltered.  It  was  collected  in  the  Sutro  Tunnel,  close  to  the  hanging  wall 
of  the  Lode  in  the  Savage  claim.  The  same  rock  is  described  in  Chapter 
III.,  slide  18,  and  its  analysis  is  given  in  the  table  following  page  151 

Results. — The  results  are  reported  chronologically,  but  with  all  correc- 
tions, including  those  based  on  subsequent  experiments.  Temperatures  are 
given  throughout  in  degrees  centigrade,  electromotive  forces  in  volts. 

From  an  inspection  of  the  tables  containing  the  results  for  the  variation 
of  the  electromotive  forces  of  bismuth-silver  and  bismuth-platinum  with 
temperature,  it  will  be  seen  that  the  relation  is  in  both  cases  so  nearly 
linear  that  it  may  at  once  be  assumed  as  such.  One  constant,  a,  only, 
therefore,  results  from  the  calculation.  Tables  I.  and  II.  contain  the  data 
for  the  calculation  of  the  thermo-electric  constant  a  for  the  triple  element 
bismuth-silver,  together  with  the  results  obtained.  T  is  the  temperature  of 
the  warmer,  t  of  the  colder  end  of  the  element,  e  the  electromotive  force 
corresponding  to  the  temperatures  of  the  respective  observations  observed 
or  calculated  as  specified,  (5(e)  finally  the  difference  between  observed  and 
calculated  results.  Two  sets  of  observations  were  made  in  order  to  ascer- 
tain to  what  extent  a  fixed  value  for  a  could  be  presumed — the  bismuth  bars 
being  cast  and  not  pressed.  In  the  calculations  preference  was  given  to 
values  of  e  corresponding  to  greater  differences  of  temperature. 

Table  I, 


No. 

t. 

T. 

ex  10= 
observed. 

ex  103 
calculated. 

«(e)xlO». 

1 

6.1 

75.1 

15.61 

15.63 

-2 

2 

6.3 

68.7 

14.13 

14.13 

±0 

3 

6.4 

63.8 

12.89 

13.00 

-11 

a~226.  5 :  106. 

4 

6.7 

57.0 

11.41 

11.39 

+2 

5 

6.9 

60.5 

9.92 

9.88 

+4 

6 

7.1 

45.0 

8.64 

8.59 

+  5 

. 

7 

7.2 

39.5 

7.32 

7.32 

+  0 

8 

7.6 

34.9 

6.25 

6.18 

+7 

9 

7.8 

30.4 

5.18 

5.12 

+6 

10 

8.3 

25.2 

3.84 

3.83 

+1 

300 


GEOLOGY  OF  THE  COMSTOCK  LODE. 
Table  II. 


No. 

e. 

T. 

ex  103 
observed. 

6X103 

calculated. 

a(e)xlO». 

1 

11.0 

73.7 

14.22 

14.22 

±0 

2 

11.3 

64.2 

11.96 

12.00 

-4 

3 

U.5 

58.7 

10.70 

10.70 

±0 

* 

11.8 

52.8 

9.30 

9.30 

±0 

a -226.  7:  10«. 

5 

12.2 

47.8 

8.10 

8.07 

+3 

6 

12.3 

42.1 

6.78 

6.76 

+2 

7 

12.5 

36.9 

5.52 

5.53 

-1 

8 

12.8 

32.2 

4.43 

4.40 

+3 

9 

13.0 

27.2 

3.19 

3.22 

:        -3 

10 

12.8 

16.4 

0.87 

0.82 

i 

Table  III.  contains  the  successive  values  of  M,  or  the  difference  of 

temperature  between  the  interior  and  exterior  of  the  rock-chamber.     It 

also  shows  the  date  of  each  observation  and  the  number  of  hours  which 

had  elapsed  since  ebullition  first  set  in.     Corrections  for  the  variation  of  a 

and  the  electromotive  force  of  the  normal  element  E  have  been  applied. 

During  the  time  covered  by  the  first  six  observations  the  water  lost  by 

evaporation  was  supplied  somewhat  intermittently;  subsequently,  however, 

as  well  as  throughout  all  succeeding  experiments,  it  was  fed  into  the  boiler 

drop  by  drop,  so  that  the  feeding  process  may  be  considered  as  practically 

continuous.     M  is  positive,  this  sign  having  been  chosen  to  indicate  that 

the  space  exterior  to  the  rock-chamber— or  the  end  of  the  thermo-element 

in  steam — is  the  hotter. 

Table  III. 


No. 

Date. 

Eonra 

t^t. 

No. 

Date. 

HOQTS. 

At. 

ft. 

h. 

1 

Dec.  10,   6 

6 

0.059 

14 

Dec.  14, 18 

42 

0.064 

2 

Dec.  10, 20 

20 

0.068 

15 

Dec.  14, 23 

47 

0.063  < 

3 

Dec.  11,  8 

32 

0.054 

16 

Dec.  15,   8 

56 

0.  062   ; 

4 

Dec.  11, 18 

43 

0.074 

17 

Dec.  15, 14 

62 

0.060 

5 

Dec.  12, 12 

60 

0.055 

18 

Dec.  15,  20 

68 

0.059 

6 

Dec,  12, 22 

70 

0.071 

19 

Dec.  15,  24 
Dec.  16,  4 

72 
76 

0.059 

7 

Dec.  13,   6 

6 

0.065 

•n 

Dec.  16,   8 

80 

0.060 

8 

Deo.  13, 10 

10 

0.005 

22 

Dec.  16, 12 

84 

0.058 

9 

Dec.  13, 14 

14 

0.062 

23 

Dec.  16, 18 

90 

0.060 

10 

Dec.  13,  IS 

18 

0.068 

24 

Dec.  16, 24 

96 

0.057 

11 

Dec.  14,  3 

27 

0.063 

V.b 

Dec.  17,   4 

100 

0.059 

n 

Dec.  14,   8 

32 

0.063 

'f» 

Dec.  17,  8 

104 

0.057 

13 

Dec.  14, 13 

37 

0.062 

EXPERIMENTS  ON  KAOLINIZATION. 


301 


Table  IV.  finally  gives  the  data  obtained  for  the  calculation  of  a  after 
the  experiments  in  Table  III.  had  been  completed,  together  with  the  results 
of  calculation.     The  nomenclature  is  the  same  as  before. 


Table  IV. 


No. 

(. 

I. 

exlO^ 
observed. 

6X103 

calculated. 

S(e)xl0». 

1 

10.0 

74.8 

14.12 

14.18 

-6 

2 

10.0 

66.7 

12.36 

12.42 

—  6 

3 

10.3 

60.7 

11.04 

11.04 

±0 

4 

10.4 

55.8 

9.95 

9.95 

±0 

0  =  2191:100. 

5 

10.5 

49.5 

8.59 

8.55 

+  4 

6 

10.7 

44.3 

7.39 

7.36 

+  3 

7 

10.8 

40.1 

6.47 

6.42 

+  5 

8 

11.0 

33.3 

4.97 

4.89 

+  8 

9 

U.2 

25.8 

3.27 

3.20 

+  7 

10 

11.8 

20.0 

1.88 

1.80 

+  8 

If  the  values  of  a  in  Tables  I.  and  II  are  compared  with  that  in  Table 
IV.,  a  difi"erence  of  about  3  per  cent,  will  be  found.  This  may  be  due  partly 
to  a  change  in  the  internal  structure  of  the  bismuth  bars,  partly  to  the  fact 
that  both  bismuth  and  silver  were  attacked  by  the  sulphur  fumes  generated 
in  consequence  of  the  presence  of  ii'on  pyrites  in  the  rock.  In  the  case  of 
bismuth  this  action  merely  produced  a  thin,  colored  coating  of  sulphide  on 
the  exterior.  The  silver,  however,  was  so  deeply  corroded  that  its  use  liad 
to  be  abandoned,  and  in  subsequent  experiments  this  metal  was  replaced  by 
platinum. 

The  data  for  ^t  show  a  difference  of  temperature  between  the  interior 
and  exterior  of  the  rock-chamber,  which  is  much  greater  than  was  antici- 
pated. Moreover,  the  consecutive  values  of  this  quantity  gradually  de- 
crease, indicating  thereby  an  apparent  increase  of  the  temperature  of  the 
rock  itself 

Tables  V.  and  VII.  contain  the  data  obtained  in  the  determination  of  a 
respectively  before  and  after  the  measurements  of  ^t  made  during  the  inter- 
mediate week.  In  Table  VI.  these  measurements  are  given,  together  with 
the  date,  barometric  height,  and  water-level,  I  (in  inches  from  the  bottom  as 
zero),  corresponding  to  each  /^t.  The  figures  for  barometric  height  were 
obtained  from  a  small  aneroid.  No  reliance  can  therefore  be  placed  on  the 
values  as  absolute,  though  the  fluctuations  are  probably  represented  with 


302 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


tolerable  faithfulness.     Besides  these  data,  the  number  of  hours  which  had 
elapsed  since  ebullition  first  set  in  are  also  given. 

Table  V. 


No. 

(. 

T. 

ex  10' 
observed. 

ex  103 
calculated. 

6(e)  X  10». 

1 

12.6 

79.7 

14.51 

14.51 

±0 

2 

15.5 

65.6 

10.79 

10.84 

-5 

3 

18.4 

61.1 

9.28 

9.24 

+4 

4 

12.7 

53.9 

8.98 

8.91 

+7 

0=216.3:106. 

5 

18.3 

53.0 

7.46 

7.51 

-5 

6 

13.0 

46.4 

7.22 

7.22 

±0 

7 

13.1 

42.9   . 

6.39 

6.45 

-6 

8 

15.6 

39.8 

5.23 

5.24 

-1 

9 

13.4 

32.9 

4.28 

4.22 

+6 

10 

15.3 

31.9 

3.52 

3.46 

+6 

Tajble  VI. 


No. 


Date. 


Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Deo. 


25,23' 
26,  S' 
26,  9' 
26, 14' 

26,  22' 

27,  3', 
27,  9' 
27, 14', 
27,  23' 
28, 10' 
28,14' 


Ht8. 


23 
27 
33 
38 
46 
51 
57 
62 
71 
82 
86 


0.098 
0.093 
0.088 
0.088 
0.083 
0.082 
0.078 
0.077 
0.072 
0.068 
0.066 


Bar.  H't. 


23.30 
23.30 


23.24 
2.3.15 
23.10 
23.06 
23.15 
23.23 
23.19 


4.9 
4.2 
4.9 
4.8 
5.0 
4.7 
4.5 


No. 


Date. 


Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec. 
Dec 
Dec. 
Dec. 
Jan. 


28,23» 
29,  5' 
29, 14' 

29,  24' 

30,  9' 
30, 18' 
30,24', 

31,  9' 
31, 17' 

1,   9*, 


Hrs. 


95 
101 
110 
120 
129 
13H 
144 
153 
161 
177 


0.067 
0.067 
0.061 
0.062 
0.062 
0.062 
0.058 
0.060 
0.055 
0.048 


Bar.  H't. 

2. 

23.12 

4.9 

■J3. 17 

4.9 

23.24 

4.8 

23.30 

4.4 

23.35 

4.2 

23.36 

4.4 

23.40 

5.2 

23.30 

4.4 

23.35 

4.0 

23.25 

4.4 

Table  vn. 


No. 

t. 

T. 

ex  103 
observed. 

exl03 
calcnlated. 

He)  X  10«. 

1 

14.1 

72.0 

12.32 

12.39 

-7 

2 

14.1 

65.8 

11.13 

10.06 

+7 

3 

14.1 

60.1 

9.92 

9.84 

+8 

4 

14.1 

54.1 

8.56 

8.56 

±0 

a=213.9:10«. 

5 

14.3 

50.3 

7.66 

7.70 

-4 

6 

14.3 

45.0 

6.54 

6.67 

-3 

7 

14.3 

40.9 

5.64 

5.69 

-5 

8 

14.3 

36.1 

4.66 

4.66 

±0 

9 

14.3 

30.9 

3.57 

3.55 

+2 

10 

13.9 

38.1 

5.18 

5.18 

±0 

A  large  difference  between  the  temperatures  of  the  interior  and  exterior 
of  the  cylinder,  the  former  being  the  smaller,  but  increasing  more  rapidly 
than  before,  is  again  apparent. 


EXPERIMENTS  ON  KAOLINIZATION. 


303 


Table  VIII.  records  an  uninterrupted  series  of  observations  made  by 
Mr.  Becker  on  the  variation  of  M  during  an  interval  of  three  weeks 
Table  IX.  contains  the  final  check  of  the  value  of  a. 


Table  VIII. 


No. 

Date. 

Hra. 

Bar.  H't. 

A«. 

I. 

No. 

Date. 

Hrs. 

Bar.  H't. 

At. 

I. 

1 

Jan.  4,21'... 

3 

23.25 

0.024 

26 

Jan.l5,  7'.... 

253 

22.83 

0.062 

4.5 

2 

Jan.   5,   S'... 

11 

23.25 

0.033 

4.2 

27 

Jan.  15, 21'.... 

267 

0.063 

4.6 

R 

Jan.  5,  21'' . . . 

27 

0.033 

4.7 

••R 

Jan  16    7' 

277 

22  94 

0.067 
0.068 
0.063 

5.0 
4.6 
4.7 

4 

Jan.  6,   7''... 
J.on.  6, 16'  . . . 

37 
46 

23.15 

0.044 
0.042 

4.4 
4.8 

29 

Jan.  16,  21'  ... . 

291 
301 

23.29 

fi 

Jan.   6,21'... 

51 

0.042 

4.8 

1  31 

Jan.  17,  21'  ... . 
Jan.  18, 10'  ... . 

7 

Jan.   7,   7'    . . 

61 

23.18 

0.042 

4.0 

{32 

328 

23.36 

0.068 

5.2 

8 
9 

Jan.  7,17'.... 
Jan.  7, 21' 

71 
75 

0.041 
0.044 

4.8 
4  3 

33 
11 

Jan.  18.  21'  ... . 
Jan.  19,  20'  ... . 
Jan.  20,   7'.... 

339 
362 

0.066 
0  063 

4.8 
4.5 
4.6 

23  30 

10 

Jan.   8,  7'.... 

85 

23.21 

0.044 

3.8 

35 

373 

23.36 

0.021 

11 

Jan.   8,15'.... 
Jan.  8,20'.... 

93 

23.50 

0.016 

4.8 

1?, 

98 

0.042 

5.3 

37 

Jan.  21, 22'  ... . 
Jan. 22,  7'.... 
Jan.  22, 22'.... 

13 
14 

Jan.   9,  7'..-. 
Jan.   9, 17'  . . . 

106 
119 

23.18 

0.045 
0.041 

4.0 
4  ■> 

38 
39 

421 
436 

0.048 

15 

Jan.  9, 21'  ... . 
Jan.  10,  7'.... 

123 

0.039 

4.6 

0.034 
0.081 

5.2 

16 

133 

23.15 

0.042 

4.5 

41 

Jan.  23,  21'  ... . 

459 

17 

Jan.  10, 19' 

145 

0.041 

4.5 

4'' 

Jan.  24,   7'.... 
Jan.  24,  21' . . . 
Jan  25    8' 

0.046 
0.018 
0  076 

4.7 
4  4 

18 
1<) 

Jan.  11, 13'  ... . 
Jan.  11, 20' 

163 
171 

0.040 
0.  050 

4.3 
4.7 

43 
'I'l 

483 
494 

2**  81 

20 

Jan.  12,   7'.... 

181 

23.20 

0.049 

4.5 

4.'i 

Jan,  25, 21'.... 

507 

0.051 

?1 

Jan.  12, 17'  ... . 

191 

0.057 

4.7 

'16 

Jan.  26,  7'.... 
Jan.  26, 21'.... 
Jan.  27,  7'.... 

517 

22  92 

0  149 

4  6 

n 

Jan.  12,  21'.... 

195 

0.055 

4.5 

47 

531 

23 

Jan.  13,   8'.... 

206 

23.12 

0.060 

4.6 

48 

541 

23.15 

0.094 

4.5 

24 
23 

Jan.  14,   7'.... 
Jan.  14, 17'.... 

229 
239 

22.90 

0.067 
0.063 

4.5 
4.9 

49 

Jan.  27, 15'.... 

549 

0.126 

Table  IX. 


No. 

(. 

T. 

exlO^ 
observed. 

ex  103 
calculated. 

6(e)  X 106. 

1 

12.8 

64.6 

10.87 

10.89 

-2 

2 

12.9 

59.5 

9.79 

9.80 

-1 

3 

12.9 

54.4 

8.78 

8.73 

+5 

a  =^10.3:  10'. 

4 

13.0 

50.1 

7.74 

7.80 

-6 

5 

13.1 

44.7 

6.65 

6.65 

±0 

6 

13.1 

40.7 

5.81 

5.81 

±0 

7 

12.8 

32.2 

4.13 

4.08 

+5 

8 

13.1 

30.1 

3.62 

3.57 

+5 

9 

12.7 

25.8 

2.75 

2.76 

-1 

10 

12.7 

22.1 

2.02 

1.98 

+4 

In  the  foregoing  determinations  of  «,  the  temperature  t  was  chosen  to 
coincide  as  nearly  as  possible  with  that  of  the  room.  Though  this  arrange- 
ment furnished  important  practical  advantages  {t  varying  but  slightly),  only 


304 


GEOLOGY  OF  THE  COMSTOOK  LODE. 


that  part  of  the  thermo-element  lying  near  the  hot  end  was  really  in  action. 
It  was  therefore  thought  desirable  to  reverse  the  element,  so  that  the  end 
which  was  formerly  in  hot  water  would  now  be  in  cold,  and  vice  versd. 
Table  X.  contains  the  results  thus  obtained. 

Table  X. 


1 

No. 

t. 

T. 

exl0» 
observed. 

«xlO» 
calculated. 

«(«)  X 10*. 

1 

13.3 

63.6 

10.61 

10.51 

±0 

2 

13.4 

57.4 

9.23 

9.19 

+■4 

3 

13.5 

45.0 

6.54 

6.58 

-4 

O  =  208.9:10«. 

4 

13.5 

40.6 

5.65 

5.66 

-1 

5 

13.5 

35.9 

4.69 

4.68 

+  1 

6 

13.5 

30.7 

3.60 

3.59 

+  1 

The  difference  between  the  values  of  a  in  Tables  IX.  and  X.  lies  within 
the  range  of  unavoidable  errors. 

In  Table  VIII.  there  is  a  difference  of  temperatures  between  the  inte- 
rior and  exterior  of  the  rock-chamber  analogous  to  that  in  preceding  tables. 
The  former  is,  as  usual,  smaller,  but  in  this  case  the  temperature  of  the 
rock  apparently  decreases  as  the  action  continues. 

Between  the  observations  No.  34  and  No.  38  there  appeared  disturbances 
of  a  kind  which  seemed  to  indicate  that  a  break  had  occurred  somewhere 
in  the  insulation.  Subsequent  inspection  showed  that  the  parts  of  the  rubber 
hose  around  the  platinum  terminals,  which  were  in  contact  both  with  air  and 
steam,  had  swollen  to  a  spongy  mass  of  many  times  their  former  bulk.  It 
is  not  improbable  that  the  wire  during  the  disturbances  mentioned  had  been 
more  or  less  perfectly  in  contact  with  the  walls  of  the  boiler,  the  doughy 
rubber  protection  having  either  given  way  or  offering  imperfect  insulation. 
Though  this  was  partially  remedied,  yet  the  last  week's  observations  are 
nevertheless  to  be  regarded  as  somewhat  suspicious,  and  were  consequently 
omitted  in  the  calculations  below. 

Discussion. — In  the  following  discussion  the  observations  in  Tables  III.  and 
VI.,  and  the  first  two  weeks  in  Table  VIII.,  are  to  be  considered.  Together 
these  data  correspond  to  an  interval  of  four  weeks.  In  the  endeavor  to 
reach  the  most  probable  conclusion  to  be  derived  from  the  large  number  of 
observations,  the  end  in  view  will  be  attained  most  speedily,  and  perhaps  most 
satisfactorily,  by  assuming  for  the  relation  between  the  variables  some  ap- 


EXPERIMENTS  ON  KAOLINIZATION.  305 

proximate  form,  and  calculating  the  constants  by  the  method  of  least  squares. 
In  the  present  case  there  is  as  much  reason  to  adopt  a  linear  form  of  func- 
tion as  any  other,  and  this  would  have  the  advantage  of  greater  simplicity. 
Denoting  the  number  of  hours  which  have  elapsed  since  the  beginning  of 
the  experiment  by  u,  let 

^t—a  +  ^u (1) 

In  this  equation  the  constant  a  is  without  great  interest.  It  simply 
denotes  the  value  of  ^t  when  ^^  is  zero,  but  is  largely  influenced  by  the 
normal  difference  of  temperature  between  the  interior  and  exterior  of  the 
rock-chamber,  i.  e.,  the  difference  which  may  be  recognized  by  an  inspection 
of  the  foregoing  tables,  and  of  Table  XIV.  /?,  however,  is  of  importance, 
representing  the  increment  of  temperature  of  the  rock  per  hour  in  conse- 
quence of  the  T.  E.  K.  It  will  be  noticed  that  j3  is  either  negative,  positive, 
or  zero,  according  as  the  process  of  kaoHnization  produces  or  absorbs  heat 
perceptibly,  or  is  without  appreciable  thermal  effect.  In  making  the  calcu- 
lation for  /?  I  had  hoped  to  be  able  to  derive  this  constant  from  the  four 
weeks'  observations,  as  a  whole.  The  problem  is  difficult,  however,  inso- 
much as  the  results  obtained  do  not  form  one  continuous  series.  The  problem 
is  not,  in  other  words,  that  of  a  single  straight  line  as  in  equation  (1),  but 
one  involving  three  straight  lines,  for  all  of  which,  however,  the  value  of 
/?  is  the  same.  Expressing  the  whole  interval  during  which  the  observations 
were  made  (four  weeks)  by  ;r\  and  regarding  the  values  of  ^t  in  Table  III. 
as  being  ordinates  of  the  component  line  whose  extreme  abscissae  are  0 
and  J ,  those  in  Table  VI.  as  belonging  to  the  line  between  J  and  I,  and  those 
in  Table  VIII.  to  the  hne  between  ^  and  ;r;  then  the  whole  line  between 
0  and  TT,  expressed  as  a  special  case  of  Fourier's  series,  would  be  represented 
by  the  equation 

Jt  =  Aisinu-\-A2sin2u-{- +  A^sinmu -}- .  .  , 

where 

A^=-}    I   *  {a^-^- ^  cp)smm<pdq)-\-     /^   {a.^  + ^  (p)smmcpdq3 

+  /„      ('^s  -\-  ^  g>)  sin.  m  (p  d  <p  >  , 
^ J 

'The  assumption  furnishes  the  constant  for  the  reduction  of  the  observations. 


306 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


and  «!,  Qfj.  <^3  are  the  intercepts  of  the  component  Hnes  on  the  axis  of  ordi- 
nates.     This  finally  leads  to 


n 


*    1    c  'i 

2_  \  ai(l  — cosw;r)  — /?— (cosm— +cosw^  +  2cosOT;r)  > 
0  m  i  4  4  2  5 


smwM, 


an  equation  which,  though  linear  with  respect  to  a.^  and  /?  and  capable  of 
further  simplification,  cannot  be  practically  utilized. 

In  view  of  this  fact,  it  was  decided  to  calculate  the  constants  ol^  and  ft 
for  each  set  of  observations  separately.  Tables  XL,  XII.,  and  XIII.  give 
the  results,  these  tables  corresponding  to  III.,  VI.,  and  VIII.,  respectively. 

Table  XI. 


No. 

u. 

A(  obs. 

HA  calc. 

Diffi 

No. 

u. 

At  Obs. 

At  calc. 

Diff: 

7 

6 

0.065 

0.065 

±0 

17 

62 

0.060 

0.061 

-1 

8 

10 

0.065 

0.065 

±0 

18 

68 

0.059 

0.060 

-2 

9 

W 

0.062 

0.065 

-3 

19 

72 

0.069 

0.060 

-1 

10 

18 

0.068 

0.064 

-H 

20 

-76 

0.060 

0.060 

+  0 

11 

27 

0.063 

0.064 

-1 

21 

80 

0.060 

0.059 

+1 

12 

32 

0.063 

0.063 

±0 

22 

84 

0.058 

0.059 

-1 

13 

37 

0.062 

0.063 

-1 

23 

90 

0.060 

0.058 

+1 

14 

42 

0.064 

0.062 

+1 

24 

96 

0.057 

0.058 

-1 

15 

47 

0.063 

0.062 

+1 

25 

100 

0.059 

0.058 

+1 

16 

56 

0.062 

0.061 

+1 

26 

104 

0.057 

0.057 

±0 

o  =-(-0.064; 

/3  =  -0.  000082  +  0.000007. 


Table  XII. 


No. 

u. 

At  obs. 

At  calc. 

Diff. 

No. 

u. 

At  obs. 

At  calc. 

Diff. 

1 

23 

0.098 

0.090 

+8 

11 

86 

0.066 

0.072 

-7 

2 

27 

0.093 

0.088 

+5 

12 

95 

0.067 

0.070 

—3 

3 

33 

0.088 

0.087 

+1 

13 

101 

0.067 

0.068 

-1 

4 

38 

0.088 

0.085 

+2 

14 

110 

0.061 

0.066 

-5 

5 

46 

0.083 

0.083 

±0 

15 

120 

0.062 

0.063 

—1 

6 

51 

0.082 

0.082 

±0 

16 

129 

0.062 

0.061 

+1 

7 

57 

0.078 

0.080 

-3 

17 

138 

0.062 

0.058 

+4 

8 

62 

0.077 

0.079 

-2 

18 

144 

0.  058 

0.057 

+1 

9 

71 

0.072 

0.076 

-4 

19 

153 

0.060 

0.054 

+6 

10 

82 

0.068 

0.074 

-6 

20 

161 

0.055 

0.052 

+3 

21 

177 

0.048 

0.048 

±0 

o=+0.096; 

P=-0.  000271  ±  0.000013. 


EXPERIMENTS  ON  KAOLINIZATION. 


307 


Table  XIII. 


ITo. 

u. 

A«  obs. 

At  calc. 

Diff. 

^0. 

u. 

At  obs. 

At  calc. 

Diff. 

1 

3 

0.024 

0.033 

-9 

18 

163 

0.040 

0.030 

-10 

2 

11 

0.033 

0.034 

-1 

19 

171 

0.050 

0.051 

-  1 

3 

27 

0.033 

0.036 

-2 

20 

181 

0.049 

0,052 

-  3 

4 

37 

0.044 

0.037 

+7 

21 

191 

0.057 

0.053 

+  4 

5 

46 

0.042 

0.038 

+  5 

22 

195 

0.055 

0.053 

+  2 

6 

51 

0.042 

0.038 

+4 

23 

200 

0.060 

0.055 

+  6 

7 

01 

0.042 

0.  039  ■ 

+  3 

24 

229 

0.067 

0.057 

+10 

8 

71 

0.041 

0.040 

+  1 

25 

239 

0.063 

0.058 

+  5 

9 

75 

0.044 

0.041 

+3 

26 

253 

0.082 

0.060 

+  3 

10 

85 

0.044 

0.042 

+2 

27 

267 

0.063 

0.061 

+  2 

11 

93 

0.043 

0.043 

±0 

28 

277 

0.067 

0.062 

+  5 

12 

98 

0.042 

0.043 

-1 

29 

291 

0.068 

0.064 

+  4 

13 

109 

0.044 

0.044 

±0 

30 

301 

0.063 

0.065 

-  2 

14 

119 

0.041 

0.045 

-4 

31 

315 

0.064 

0.066 

-  2 

15 

123 

0.039 

0.046 

-7 

32 

328 

0.063 

0.068 

+  1 

16 

133 

0.042 

0.047 

-5 

33 

339 

0.066 

0.069 

-  3 

17 

145 

0.042 

0.048 

-6 

34 

362 

0.063 

0.071 

-  8 

a= +0.033; 

^  =+0.000106  ±  0.000006. 


The  constants  /?  in  these  tables  are,  however,  of  inconvenient  magni- 
tude, and  it  will  be  more  expedient  to  represent  these  quantities  on  the 
scale  of  a  year.  Let  ^T,  then,  denote  the  apparent  increase  of  the  tem- 
perature of  the  rock  in  the  apparatus  per  year,  the  variation  being  sup- 
posed to  have  continued  during  the  whole  of  this  time  in  the  same  manner 
as  during  the  time  of  observation.  Then  from  Tables  XI.  and  XII.,  which, 
together,  comprehend  an  interval  of  two  weeks, 

^r  =  -|-l°.5±0°.l; 

and  from  Table  XIII.,  corresponding  to  the  same  interval, 

JT  =  —  0°.dzkO°.l. 

These  figures  express  the  final  result  of  the  investigation.  They  indi- 
cate that,  as  far  as  these  experiments  go,  it  would  be  about  equally  correct 
to  assume  a  positive  or  a  negative  thermal  effect  from  the  action  of  aqueous 
vapor  on  the  rock;  and  that  for  the  present,  at  least,  a  thermal  effect  may 
be  assumed  to  be  absent. 

By  comparing  corresponding  values  of  a  in  the  tables  above,  it  becomes 
evident  that  the  changes  in  the  values  of  ^t  cannot  in  any  way  be  referred 
to  the  thermo-element;  nor  is  there,  in  the  results  taken  as  a  whole,  an  effect 


308 


GEOLOGY  OP  THE  COMSTOCK  LODE. 


due  to  the  variation  of  the  barometric  pressure  or  to  thewater  level  appa- 
rent. 

A  series  of  experiments  made  with  the  rock-chamber  empty,  the  rest 
of  the  apparatus  remaining,  however,  as  before,  gave  the  following  results: 

Table  XIV. 


No. 

Sate. 

Hrs. 

At. 

No. 

Date. 

Hrs. 

At 

1 

Dec.  17,  ll'.S 

0.0 

0.020 

1 

Dec.  17, 15'.0 

0.0 

0.019 

2 

Dec.  17,  12'.  0 

0.5 

0.022 

2 

Dec.  17,  IS'.S 

0.5 

0.020 

3 

Dec.  17,  12'.5 

1.0 

0.024 

3 

Dec.  17,  le'.O 

1.0 

0.029 

4 

Dec.  17,  13'.  5 

2.0 

0.028 

i 

Dec.  17, 17''.0 

2.0 

0.031 

The  interval  of  time  covered  by  these  experiments  is,  of  course,  too 
small  to  justify  any  confidence  in  the  constants  which  might  be  derived 
from  them.  They  are,  however,  sufficient  to  show  that  Jt  undergoes 
changes  analogous  to  those  noted  in  the  preceding  pages.  It  probably  fol- 
lows, therefore,  that  the  final  results  may  be  regarded  as  giving  an  estimate 
of  the  degree  of  accuracy  attainable  by  the  method  in  its  present  shape. 
The  chief  source  of  error  is  the  fact  that  the  apparatus  does  not  maintain 
the  constancy  of  temperature  necessary.  It  is  apparently  impossible  by 
means  of  it  to  heat  the  large  mass  of  rock  to  the  same  temperature  through- 
out. Furthermore,  the  thermometer  employed  is  neither  in  sufficiently  inti- 
mate contact  with  the  rock,  nor  are  the  junctures  placed  in  circumstances 
as  nearly  identical  as  is  desirable.  Finally,  I  am  inclined  to  infer  that  a 
stationary  thermal  condition  was  not  reached  in  the  experiments.  Although 
this  supposition  accounts  for  only  a  part  of  the  anomalies  met  with,  it  will 
nevertheless  be  necessary  in  future  researches  to  extend  the  time  of  each  set 
of  observations  considerably  beyond  the  duration  of  the  above  experi- 
ments. I  omit  a  detailed  discussion  of  these  matters,  however,  as  a  further 
study  of  the  subject  is  intended. 


CHAPTER    X. 
ON  THE  ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 

BY    CAEL    BARUS. 

GENEEAL  STATEMENT. 

In  1830  R.  W.  Fox  communicated  to  the  Royal  Society  a  paper  which 
contained  the  results  of  a  careful  experimental  study  of  the  possible  electric 
activity  of  ore  bodies.  From  this  time  until  1844  the  matter  was  discussed 
with  some  enthusiasm  by  Fox  and  Henwood,  in  England,  and  by  von  Strom- 
beck  and  Reich,  in  Germany.  After  the  publication  of  Reich's  second  paper 
(1844),  however,  further  research  seems  to  have  been  altogether  abandoned; 
at  least  I  have  not,  with  some  pains,  been  able  to  find  anything  that  has  a 
bearing  on  the  subject.^  This  is  all  the  more  remarkable,  as  the  general 
line  of  investigation  had  already  taken  a  promising  direction.  It  would 
also  have  been  supposed  that  Thalen's^  work  would  have  given  the  matter  a 
fresh  impetus. 

With  the  present  investigation  (undertaken  at  the  suggestion  of  Mr. 
Becker^)  the  question  of  a  relation  between  local  currents  and  ore  bodies 
is,  as  it  were,  resuscitated,  so  that  a  general  review  of  the  development  which 
it  had  attained  previous  to  its  abandonment  seems  pertinent.     -^ 

'See,  also,  "Revue  des  Progrfes  r^centes  de  I'Exploitation  des  Mines,  etc.,  par  M.  Haton  delaGou- 
pillifere,  Ingen.  en  chef  des  Mines,  Professeur,  etc.,"  in  the  Anuales  des  Mines,  T.  XVI.,  p.  6,  1879. 

^R.  Thalen;  v.  dela  Goupilliere,  I.  c. :  "Ou  trace  des  lignes  d'^gale  intensitd,  qui  dans  le  voisi- 
nage  d'un  glte  prennent  une  forme  caract^ristique  consistant  en  deux  systfimes  de  courbes  ferm^es,  con- 
ccntriques,  autour  de  deux  foyers  assez  nettement  indiqu6s." 

3Cf. :  First  Annual  Report  of  the  U.  S.  Geolog.  Survey,  p.  46,  1880. 

(309) 


310  GEOLOGY  OF  THE  COMSTOCK  LODE. 


BEIEF  EEVIEW  OF  THE  WOEK  OF  PEEVIOUS  IISTVESTIGATOES. 

Fox/  in  his  original  experiments,  secured  electric  contact  with  the  vein 
by  wedging  copper  plates  against  it.  These  were  put  in  connection  with  a 
galvanometer  by  copper  wire.  Earth  currents,  if  present,  entered  the  wire 
at  one  end,  passing  through  the  galvanometer  and  finally  back  into  the  earth 
at  the  other. 

As  a  general  result  of  his  investigation  Fox  found  that  the  intensity 
and  direction  of  the  currents  bore  no  relation  to  the  cardinal  points,  but 
could  be  explained  by  a  consideration  of  the  distribution  of  ores.^  Between 
two  points  of  a  continuous  vein  on  the  same  level  no  cuiTent  was  observa- 
ble; but  when  the  points  tapped  were  on  different  levels,  or  when  there 
intervened  between  them  an  area  of  barren  rock  (horse),  or  when  two  appar- 
ently distinct  veins  were  connected,  the  effect  was  invariably  decisive.  At 
times  the  currents  were  so  powerful  as  to  throw  the  needle  of  his  by  no  means 
delicate  galvanometer  (3J-inch  needle  in  twenty-five  turns  of  wire)  several 
times  around  the  circle.  After  enumerating  a  number  of  facts  with  reference 
to  the  relative  position  of  the  veins.  Fox  remarks  that  "many  of  the  phe- 
nomena referred  to  bear  a  striking  resemblance  to  common  galvanic  combi- 
nations, and  the  discovery  of  electricity  in  veins  seems  to  complete  the 
resemblance."  In  other  parts  of  this  paper,  however,  he  expresses  the  opinion 
that  "mineral  veins  and  internal  heat  are  connected  with  electric  action," 
and,  moreover,  anticipates  greater  effects  with  increasing  heat  and  depth. 

The  experiments  of  v.  Strombeck^  were  made  at  Werlau  and  Holzappel 
on  a  large  vein,  in  which  quartz,  blende,  galena,  copper-pyrites,  and  tetra- 
hediite  occurred  in  irregular  distribution,  and  are  distinguished  by  the  care 
with  which  all  known  sources  of  error  were  avoided.  Contact  was  secured 
by  drilling  into  the  vein  holes  2  to  3  inches  in  depth,  into  which  the  ends  of 
the  wire,  spirally  wrapped  and  held  in  position  by  a  cork,  were  inserted.    In 

'E.  W.  Fox,  "On  the  electro-magnetic  properties  of  metalliferous  veins  in  the  mines  of  Corn- 
wall."   Phil.  Trans.,  II.,  p.  399,  1830. 

=  Galena,  copper,  and  iron  pyrites  were  the  minerals  met  with. 

»A.  y.  Stronibeck,  "Ueber  die  von  Herrn  Fox  angestellten  Untersuchungen  in  Bezug  auf  die 
elcctro-maguetischen  Aenasernngeu  der  Metallgange."    Karsten's  Archiv.,  VI.,  431,  1633. 


ELECTEICAL  ACTIVITY  OF  ORE  BODIES.  311 

other  respects  the  method  of  research  was  identical  with  that  of  Fox. 
V.  Strombeck  made  a  large  number  of  experiments,  but  was  unable  to  detect 
any  traces  of  electric  excitation,  and,  consequently,  concludes  that  Fox's 
results  are  not  applicable  to  veins  generally,  and  that  even  in  Cornwall 
the  matter  requires  further  consideration. 

In  1834  Fox  again  resumed  his  experiments,  with  special  reference  to 
the  objections  which  had  been  raised  against  the  validity  of  his  results.^  It 
having  been  mooted  that  the  currents  observed  might  in  some  way  owe  their 
origin  to  the  copper  contact-plates,  he  showed  that  by  replacing  these  by 
plates  of  zinc  the  results  remained  unaltered.  This  was  the  case  even  when 
terminals  of  copper  and  zinc  were  used  simultaneously.  It  was,  moreover, 
immaterial  whether  the  contact  was  produced  by  plates  or  whether  the  ends 
of  the  wire  only  were  pressed  against  the  vein.  By  inserting  a  copper-zinc 
couple  into  his  circuit  Fox  found  that  its  effect  was  in  some  cases  nearly,  in 
others  decidedly,  overbalanced  by  the  lode  currents.  Finally,  in  the  interval 
of  four  years  which  had  elapsed  between  these  and  his  former  experiments 
the  direction  of  the  currents  had  remained  unchanged. 

In  a  subsequent  paper  Fox^  endeavors  to  classify  minerals  with  refer- 
ence to  their  electrical  properties.  A  table  of  conductivities  is  contained  in 
his  original  paper. 

In  the  Skeers  lead  mine,  near  Middleton,  Fox^  obtained  but  feeble 
currents;  at  the  Coldberry  mine,  in  the  same  locality,  they  were  absent  alto- 
gether. Lead  mines  do  not  in  general  give  evidence  of  electrical  action 
comparable  to  that  of  copper  mines — a  circumstance  which  Fox  refers  to 
the  positions  of  their  ores  in  his  scale. 

Henwood'.s*  experiments  were  made  on  a  larger  scale  (at  times  as 
much  as  600  fathoms  of  copper  wire  were  employed),  but  otherwise  in  a  way 

'E.  W.  Fox,  "Account  of  some  experiments  on  the  electricity  of  the  copper  vein  in  Huel  Jewel 
mine."    Rep.  Br.  Assoc,  1834,  p.  572. 

-R.  W.  Fox,  "Note  on  the  electric  relations  of  certain  metals  and  metalliferous  minerals."  Phil. 
Tr.an8.,  I.,  p.  39,  IS-IS. 

"R.  W.  Fox,  "  Report  on  some  experiments  on  the  electricity  of  metallic  veins,  etc."  Rep.  Br. 
Assoc,  p.  133,  1837. 

«W.  J.  Hen  wood,  "Surles  conrants  ^lectriques  observes  dans  lesfilons  de  Comouailles,"  Annaleg 
des  Mines,  [3],  XI.,  p.  585,  1837. 


312  GEOLOGY  OF  THE  COMSTOCK  LODE. 

analogous  to  that  of  Fox.  They  contain  a  thorough  corroboration  of  the 
results  of  the  latter.  He,  moreover,  insists  that  currents  are  only  obtained 
in  the  case  where  the  points  tapped  are  in  vein  matter,  being  most  decisive 
for  copper  pyrites,  vitreous  and  black  copper  ore,  galena  and  blende;  that 
between  points  in  barren  rock  electric  action  is  altogether  absent.  After  a 
number  of  theoretical  considerations — to  which  the  paper  is  largely  devoted — 
he  concludes  that  the  currents  are  probably  of  thermo-electric  origin,  and 
that  they  are  certainly  purely  local. 

Some  time  after,  all  of  Fox's  experiments  were  again  repeated  and  the 
results  confirmed  throughout  by  Reich.^  Although  the  heating  of  one  of 
the  points  of  contact  in  the  case  where  both  were  applied  to  the  same  vein 
produced  a  decided  thermo-electric  effect,  quantitatively  this  was  so  small  as 
to  furnish  grounds  against  Henwood's  hypothesis.  Reicli  is  convinced  that 
Fox's  currents  are  hydi-o-electric  phenomena.  When  a  point  in  ore  was  con- 
nected with  one  in  rock,  the  currents  were  not  only  much  smaller — proba- 
bly on  account  of  the  greater  resistance  in  this  case — but  if  plates  of  copper 
and  zinc  were  used  together  as  terminals,  a  commutation  of  these  invariably 
produced  a  corresponding  change  in  the  direction  of  the  current. 

In  Fox's  last  paper^  on  the  subject,  the  effect  of  the  contact  plates  is 
again  carefully  considered.  But  even  with  one  terminal  of  zinc,  the  other 
of  copper,  "the  current  continued  to  deflect  the  needle  from  50°  to  60°, 
notwithstanding  that  any  action  between  the  copper  *  *  «  *  and  the 
zinc  *  *  *  *  if  it  had  existed  would  have  been  in  the  opposite  direction 
and  have  tended  more  or  less  to  counteract  the  influence  of  the  actual  cur- 
rent." The  galvanometer  referred  to  consisted  of  forty-eight  turns  of  brass 
wire  wrapped  around  a  2-inch  needle,  on  a  pivot.  The  lode  current  in  a  case 
observed  was  found  to  remain  constant  for  a  period  of  eight  months.  Toward 
the  end  of  the  paper  mention  is  made  of  experiments  in  which  one  or  both 
terminals  were  in  rock.     In  this  case  the  results  were  similar  to  those  of 


IF.  Eeich,  "NotizUber  elektrische  Strome  auf  Erzgaugen."    Pogg.  Ann.,  XLVIII.,  p.  287,  1839. 
°E.  W.  Fox,  "Some  experiments  on  subterranean  electricity,  made  at  Pennance  mine  near  Fal- 
month."    Pbil.  Mag.,  [3],  XXIII.,  pp.  457  and  491,  ie43. 


ELECTEICAL  ACTIVITY  OP  ORE  BODIES.  313 

Reich,  "there  being  still  a  tendency  to  deflection."  The  exchange  of  ter- 
minals of  different  metals  also  produced  a  change  in  the  direction  of  the 
current. 

In  the  next  year  Reich^  published  his  second  paper,  undertaken  with 
the  especial  object  of  studying  more  closely  the  currents  probably  existing 
in  the  rocks  surrounding  the  vein.  His  idea  was  that  lode  currents  are 
produced  by  the  contact  of  the  different  ores  in  the  deposit,  the  rock  which 
separates  them  more  or  less  completely  one  from  another  performing  the 
function  of  the  liquid  of  an  ordinary  galvanic  couple.  As  Fox's  method  of 
obtaining  contacts  with  the  earth  was  inapplicable,  Reich  had  holes  (12 
inches  deep)  drilled  in  the  rock,  into  which  dilute  sulphuric  acid  was  poured. 
Strips  of  copper  foil  plunged  into  the  acid  and  connected  with  the  ends  of  a 
copper  wire  completed  the  circuit.  Currents  were  obtained  when  at  least 
one  point  was  near  ore;  they  were  completely  absent  when  both  points  were 
in  barren  rock.  Though  the  deflections  of  the  needle  ranged  from  2°  to 
30°,  they  seemed  to  obey  no  general  law.  The  results  are,  moreover,  diffi- 
cult of  interpretation,  because  the  needle  does  not  discriminate  between 
high  and  low  grade,  or  between  base  and  noble  minerals,^  the  deflection 
being  a  function  of  both  the  quality  and  the  quantity  of  the  electrically 
active  material.  Reich's  mode  of  operation  was  derived  from  a  considera- 
tion of  the  currents  of  a  galvanic  cell  in  action.  The  paper'  is  interesting 
and  the  reader's  attention  is  especially  called  to  it.  I  shall  have  occasion 
to  consider  it  again  below. 

The  reader  is  finally  referred  to  the  Proceedings  Roy.  Soc.  Lend.,  III., 
p.  123,  1832,  and  IV.,  p.  317,  1841,  which  were  not  at  my  disposal. 

Remarks  on  the  foregoing. — Froui  1830  Until  1844,  therefore,  the  papers  in 
hand  offer  little  more  than  a  criticism  of  Fox's  original  investigation.  In 
1844,  with  the  publication  of  Reich's  second  paper,  in  which  the  idea  that 
if  local  currents  due  to  ore  bodies  are  present  at  all  they  must  be  discover- 
able in  the  rocks,  was  the  basis  of  research,  a  second  step  may  be  considered 

'F.  Reich,  "Versucte  uber  die  Aufsuchung  von  Erzen  mittelst  des  Schweiger'schen  Multiplica- 
tors."    Berg-  u.-  huttenmiinn'sche  Ztg.,  [3],  pp.  342-346,  386-390,  1844. 

"The  term  "mineral"  wherever  used  throughout  this  chapter  is  intended  to  refer  to  those  of  the 
heavy  metals  only — to  those  in  short  in  which  we  may  expect  to  find  metallic  properties. 

'See,  also,  B.  v.  Cotta,  "  Erzlagerstiitten,"  Vol.  I. 


314  GEOLOGY  OP  THE  COMSTOCK  LODE. 

as  having  been  made.  It  is  to  be  regretted  that  in  none  of  the  papers  is 
there  even  an  attempt  toward  fully  describing  the  phenomena  quantita- 
tively. Generally,  conclusions  are  drawn  from  the  deflection  of  a  galvano- 
meter needle  without  sufficient  consideration  of  the  very  probable  variation 
of  the  resistance  of  different  circuits.  The  experiments  are,  moreover,  made 
individually,  not  in  series  or  with  reference  to  any  definite,  preorganized 
plan.  Insomuch,  however,  as  most  of  the  work  was  done  when  methods  of 
electric  measurement  were  still  in  their  infancy,  these  matters  are  not  to  be 
mentioned  to  the  disparagement  of  the  authors.  In  fact,  the  reader  is  sur- 
prised at  the  broad  view  usually  taken,  at  the  cautiousness  with  which 
hypotheses  are  stated,  and  at  the  number  of  details  and  chances  of  error 
which  are  considered. 


HYPOTHESIS  UNDEELYING  THE  PRESENT  INVESTIGATION. 

There  can  be  little  doubt  that  the  hypothesis  which  ascribes  to  ore- 
currents  a  hydro-electric  origin  is  perfectly  correct.  Fox  and  Reich  them- 
selves found  in  the  case  of  terminals  of  copper  and  zinc  used  together,  the 
points  tapped  being  in  rock,  that  currents  resulted,  the  direction  of  which 
changed  with  an  exchange  of  the  terminals.  I  have  actually  measured  the 
electromotive  force  in  action  under  these  circumstances  (see  page  322),  and 
found  it  of  the  same  order  as  that  produced  by  combining  these  metals  with  a 
liquid  in  the  form  of  a  galvanic  element.  If,  then,  there  are  also  ores  which 
possess  the  electric  properties  of  metals — and  that  this  is  the  case  Fox*  went 
to  some  trouble  to  show — the  possibility  of  ore-currents  due  to  hydro- 
electric action  follows  as  an  immediate  consequence.  These  currents  will 
in  general  have  an  origin  analogous  to  those  technically  known  as  "local 
currents  "  in  batteries,  while  at  times  they  may  even  be  due  to  the  occurrence 
of  a  complete  natural  battery.  Thermo-electric  hypotheses  are  unnatural, 
insomuch  as  with  the  temperatures  met  with,  even  in  the  Comstock,  it 
would  be  necessary  to  assume  values  for  thermo-electric  power  which, 
in  comparison  with  those  of  known  substances,  are  abnormally  large.     Such 

'K.  W.  Fos,  Phil.  Trans.,  1.,  p.  39,  1835. 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES.  315 

a  speculation  is,  therefore,  remote,  artificial,  and  forced,  and,  in  cases  where 
there  is  a  better  hypothesis,  deserves  only  very  secondary  consideration. 

Suppose  now  that,  in  connection  with  an  ore  body,  with  reference  to 
which  experiments  are  being  conducted,  electric  action  actually  does  occui-. 
In  the  consideration  of  these  currents  we  are  at  once  confronted  by  the  impor- 
tant fact  that  insomuch  as  electric  action  has  been  going  on  for  an  indefinite 
period  of  time  the  currents  must  have  become  constant  both  in  intensity 
and  direction,  and  that  therefore  the  equipotential  surfaces  corresponding  to 
this  flow  will  have  fixed  and  probably  well-definable  positions. 

In  view  of  the  fact  that  with  most  geological  readers  the  consideration 
of  electric  phenomena  will  be  merely  an  incidental  matter,  it  may  be  well  to 
be  more  explicit  than  would  otherwise  be  necessary.  By  far  the  greater 
number  of  electrical  phenomena  can  be  explained  by  regarding  electricity 
as  in  the  nature  of  an  incompressible  fluid.  The  analogy  is,  in  fact,  very 
complete,  and  extends  even  into  further  detail  than  need  be  noticed  here. 
We  speak  of  a  liquid  as  having  a  tendency  to  flow  from  a  higher  to  a  lower 
level;  of  electricity,  as  flowing  from  an  equipotential  of  greater  to  one  of 
less  value.  In  the  former  case  the  "levels"  are  approximately  spheroidal 
surfaces — "geoids" — parallel  to  the  normal  surface  of  the  earth;,  in  the  lat- 
ter they  may  be  closed,  or  may  extend  to  infinity;  they  may  be  quite  simple 
or  exceedingly  complex.  In  order  to  exhibit  the  topography  of  a  country  in 
detail,  it  may  be  repi'esented  graphically  by  the  aid  of  a  series  of  equidistant 
earth  levels.  In  electricity  an  analogous  problem  is  similarly  solved,  those 
surfaces  being  chosen  for  which  the  potential  value  from  surface  to  surface 
increases  by  a  definite  amount.^  If  a  reservoir,  the  water  in  which  is  con- 
stantly at  a  level,  j),  be  joined  by  a  pipe  with  one  in  which  the  water-level 
is  constantly  g  (both  j)  and  g-  being  measured  vertically  upwards  from  some 
fixed  datum,  and  p'^q),  the  quantity  of  liquid  traversing  any  right  section 
of  the  pipe  in  the  unit  of  time  would  ccet.  par.  be  dependent  on  the  dimensions 
of  the  latter  and  upon  p — q.  If  a  point  on  an  equipotential  of  the  value  p 
be  connected  by  a  thin  wire  with  a  point  on  one  of  the  value  q,  analogous 
remarks  may  be  made  with  reference  to  the  quantity  of  electricity  (/)  flowing 

'Neither  level  nor  potential  imply  the  presence  of  matter  or  of  electricity,  respectively,  at  a 
given  point. 


3 1 6  GEOLOGY  OF  THE  COMSTOCK  LODE.       ^K^m 

through  any  right  section  of  the  wire  in  the  unit  of  time.  Now  HHB 
it  is  upon  I  that  the  deflection  of  a  magnetic  needle  surrounded  H^^l 
by  a  coil  of  wire,  the  plane  of  the  windings  being  vertical  and  HHB 
parallel  to  the  needle,  ccet.  par.,  depends;  whence  it  follows,  even  HBH 
if  the  same  arrangement  of  coil  and  needle  were  used  throughout,  H^H 
that  the  deflection  just  mentioned  would  contain  an  incidental  H^^H 
element;  in  other  words,  that  it  depends  upon  the  means  which  H^^l 
have  been  adopted  to  efi"ect  the  connection  between  the  equipo-  HlH 
tentials  p  and  q.  I^HI 

Returning  to  the  problem  in  hand,  it  will  be  found  that  the  HBH 
mere  measurement  of  deflections  would  be  of  but  little  avail.  An  H^H 
effort  must  be  made  to  determine  the  values  of  p  and  q  at  the  ^H^h 
points  tapped  by  the  ends  of  a  wire.  These  quantities,  more-  ^HBb 
over,  are  particularly  significant,  insomuch  as  the  potential  at  ^^^H 
any  given  point  in  the  vicinity  of  the  ore  body  depends  princi-  ^^^H 
pally  upon  the  character  and  distribution  of  the  electrically  active  B^9 
ore-matter,  and  of  the  rock  surrounding  it,  or  wholly  on  con-  H^H 
ditions  fixed  by  nature.  Hence,  instead  of  seeking  for  the  ore  HHH 
body  itself,  an  attempt  will  be  made  to  add  to  the  few  clews  H^H 
available  to  the  prospector  by  investigating  some  characteristic  H^H 
variation  of  the  potential  at  consecutive,  similarly  disposed  points,  HH| 
as  indicating  proximity  to  it.  But  what  has  been  said  of  p  and  HHII 
q  applies  equally  well  to  p—q,  which  latter  quantity  is,  moreover,  H^H 
easily  measurable,  either  directly  (electrometrically,  or  by  cer-  ^^^H 
tain  galvanometric  methods)  or  indirectly,  by  the  determination  ^^^| 
of  the  magnitude  of  deflection  of  the  needle  described  above,  ^^^H 
under  known  conditions,  p — q  is  technically  called  electromotive  H^M 
force.  I^B 

To  an  observer  the  equipotentials  are  accessible  for  measure-  H^H 
ment  either  on  the  surface  or  in  those  places  where  drifts  pene-  H^H 
trate  them.  Let  a,  v,  r.  Fig.  22,  be  a  line  lying  either  upon  or  H^H 
within  the  surface  of  the  earth.  Suppose  the  electromotive  forces  H^H 
be  measured  between  a  point  a,  and  consecutive  points  /?,  y,  6  H^H 
.  .  .  fi,  V,  $,  .  .  .  .  a,  T,  V,  .  .  .  taken  at  convenient,  approximately  H^H 
equal,  distances  apart.     The  points  ju,  v,  ^  .  .  .  are  supposed  to     fig.  22. 


ELECTEICAL  ACTIVITY  OF  OEE  BODIES.  317 

be  near  the  ore  body,  whereas  a,  /3,y  .  .  .  and  g,  t,  v  .  .  are  remote  from  it. 
As  I  shall  frequently  have  occasion  to  refer  to  the  point  a  in  contradistinc- 
tion to  the  remaining  points  /?,  7,  5, .  .  .  (>,  t,  t» . . ,  I  will  throughout  this  chap- 
ter refer  to  the  former  under  the  name  permanent  contact  (P.  C),  while  to 
any  of  the  others  the  name  temporary  contact  ( T.  C.)  will  be  applied.  Then 
will  the  electromotive  force  (e)  between  P.  C.  and  any  T.  C.  in  general  vary 
with  the  distance  (x)  between  these  points.  This  relation  will  usually  be  so 
complex  as  not  to  be  easily  expressible  by  mathematical  means,  but  it  can 
nevertheless  be  indicated  symbolically  by 

e=f{x). 

If,  however,  x  is  supposed  to  increase  from  zero  (in  which  case  P.  C. 
and  T.  C.  coincide)  to  the  value  it  has  for  some  remote  point,  v,  then  as  a 
field  of  electrical  activity  is  encountered  in  the  neighborhood  of  /',»',  ^, 
f(x)  must  pass  through  a  single  maximum  or  minimum,  or  a  number  of  them. 
It  is  therefore  toward  a  characteristic  variation  of  this  kind  that  we  must 
look  in  endeavoring  to  define  a  position  of  greatest  proximity  to  the  ore 
body.  Analogously,  though  less  generally,  it  may  be  stated  that  the  incre- 
ment of  potential  due  to  successive  increments  of  distance  a  /?,  /?  y,  y  S,  etc., 
will  be  small  except  in  the  neighborhood  of  the  ore  body.  This  is  probably 
the  idea  which  Reich  had  in  mind,  and  which  he  must  have  come  upon  had 
he  followed  out  the  line  of  his  argument  to  its  consequences. 

I  will  add  here  that  local  difficulties  did  not  permit  me  actually  to  pass 
linearly  through  an  ore-region.  I  had  to  content  m3'^self,  therefore,  with  a 
progress  from  the  latter  into  barren  rock. 


EXPERIMENTS  MADE  IN  SOME  OF  THE  MINES  ON  THE  COMSTOCK. 

Method. — Experiments  were  commenced  in  the  Consolidated  Virginia,  Cal- 
ifornia and  Ophir  mines,  the  line  at  times  extending  into  Union  and  Mexican 
ground. 

From  the  work  of  previous  investigators  I  was  naturally  led  to  expect 
currents  due  to  electromotive  forces  of  considerable  magnitude,  and  as  a 
consequence,  was  satisfied  with  a  method  of  obtaining  contact  with  the  vein 


318 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


in  which  the  electromotive  force  due  to  the  terminals  alone  was  not  greater 
than  a  few  hundredths  of  a  volt.  Bright  steel  gads,  to  the  tops  of  which  pieces 
of  thick  copper  had  been  firmly  fastened,  were  especially  convenient  for 
this  purpose,  as  they  could  be  driven  into  the  vein  or  again  withdrawn  from 
it  expeditiously.  These  gads  were  from  8  to  10  inches  long  and  about  one  inch 
in  diameter  at  the  head,  from  which  they  tapered  gradually  to  a  point.  As 
it  would  be  repeatedly  necessary  to  use  them  in  places  where  the  earth  was 
naturally  moist,  the  question  arose  whether  it  might  not  be  desirable  in  all 
the  experiments  to  moisten  the  rock  around  the  gads  at  once.  Accordingly, 
two  sets  of  experiments,  the  results  of  which  are  contained  in  Tables  I.  and 
XL,  were  made,  the  former  above  the  surface,  the  latter  below. 

Two  suitable  positions  in  rock  free  from  mineraP  matter  having  been 
selected,  the  gads  were  driven  and  the  circuit  completed.  Measurements  of 
resistance  and  electromotive  force  were  then  made.  The  gads  were  now 
exchanged  and  the  measurements  repeated,  and  so  on.  The  relative  posi- 
tion of  the  gads  to  an  observer  facing  them  is  indicated  in  the  second  column 
of  the  tables.  Resistance  (  W)  in  ohms  and  electromotive  force  (e)  in  volts 
are  given  in  the  third  and  fourth  columns,  respectively.  The  last  column 
shows  the  direction  of  the  current,  arbitrarily  called  "_j-"  when  flowing  in 
one  way,  "  — "  when  flowing  in  the  opposite. 

Table  I. — Experiments  inade  on  south  side  of  Bullion  Ravine. 

[Gads  driven  into  quartz  seams  between  walls  of  diorite,  about  10  feet  apart.    Seams  natnrally  somewhat  moist.] 


Gads  dry. 

Gads  wet. 

No. 

Position 
of  the 
gads. 

w. 

€. 

Direction 

of  the 

current. 

No. 

Position 
of  the 
gads. 

w. 

e. 

Direction 
of  the 
current. 

1 

I,  n 

7600 

0.03 

+ 

1 

n,  I 

1560 

0.01 

■+ 

2 

II,  I 

6300 

0.09 

+ 

2 

I,  n 

1260 

0.02 

+ 

3 

I,  n 

4300 

0.01 

_ 

3 

n,  I 

1280 

0.02 

+ 

4 

n,i 

4500 

0.06 

+ 

4 

I,  n 

1200 

0.01 

+ 

5 

I,  n 

3700 

0.00 

— 

5 

n,  I 

1210 

0.01 

- 

6 

n,i 

3400 

0.01 

+ 

6 

1,11 

1200 

0.01 

- 

7 

i,n 

3200 

0.00 

+ 

7 

II,  I 

1230 

0.01 

+ 

8 

i,n 

1240 

0.01 

- 

9 

n,  I 

1240 

0.04 

+ 

'  See  note,  page  313. 


ELECTKICAL  ACTIVITY  OF  OEE  BODIES. 


319 


Table  II. — Experiments  in  the  Con,  Virginia  and  California^  1750-foot  level, 

[Gada  driven  into  rock,  as  free  from  mineral  matter  as  possible,  about  8  feet  apart.] 


GadB  dry. 

Grade  vret. 

No. 

Position 
of  tbe 
gads. 

w. 

€. 

Direction 
of  the 
current. 

No. 

Position 
of  the 
gads. 

w. 

€. 

Direction 
of  the 
current. 

Dale. 

1 

n,i 

6000 

0.04 

+ 

1 

II,  I 

550 

0.03 

_ 

Sept.  24, 1880. 

2 

I,  n 

3700 

0.04 

+ 

2 

I,  n 

500 

0.03 

- 

Sept.  24, 1880 

3 

n,  I 

2800 

0.02 

+ 

3 

II,  I 

450 

0.01 

- 

Sept.  24, 1880 

4 

I,  II 

2200 

0.02 

+ 

4 

I,  II 

400 

0.02 

- 

Sept.  24, 1880 

5 

n,  I 

1870 

0.02 

- 

5 

II,  I 

380 

0.01 

_ 

Sept.  24, 1880 

6 

I,  II 

1380 

0.01 

+ 

6 

HI 

390 

0.01 

+ 

Sept.  25, 1880 

7 

U,  I 

1030 

0.03 

+ 

7 

I,  n 

270 

0.03 

+ 

Sept.  25, 1880 

8 

I,  n 

1060 

0.01 

- 

8 

n,  I 

280 

0.01 

+ 

Sept.  25, 1880 

9 

I,  II 

260 

0.03 

- 

Sept.  25, 1880 

10 

n,  I 

270 

0.01 

- 

Sept.  25, 1880 

The  results  are  highly  in  favor  of  wet  gads.  By  their  use  a  very 
marked  diminution  of  resistance  is  effected  without  increasing  the  values  of 
£.  The  direction  in  which  e  acts  follows  no  observable  law,  probably  being 
conditioned  by  the  electrical  difference  of  the  gads  and  by  effects  of  polar- 
ization due  to  the  introduction  of  a  Daniell. 

Analogous  experiments  were  also  made  with  copper  and  zinc.  These 
metals  were  used  in  the  form  of  strips  cut  from  sheets.  Each  strip  was 
bent  around  the  small  end  of  a  slightly  conical  stick  of  wood  about  one 
foot  in  length.  The  plug  was  then  firmly  driven  into  a  hole  previously 
drilled  for  the  purpose,  in  such  a  way  as  to  force  the  metal  into  thorough 
contact  with  the  rock  Table  III.  gives  the  results,  the  notation  being  the 
same  as  that  used  in  Table  I. 

Table  III. — Experiments  in  the  Con.  Virginia  and  California,  nsOfoot  level. 

[Pings  abont  10  feet  apart  in  moist  clay  seams,  repeatedly  exchanged  as  indicated.] 


Copper  pings,  wet. 

Zinc  plugs,  wet.          ] 

No. 

Position 
of  plugs. 

«. 

No. 

Position 
of  plugs. 

e. 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 

I,  n 
n,  I 

I,  II 

II,  I 
I,  n 
n,  I 
i,ir 
n,  I 

I,  II 

II,  I 

+0.02 
+0.02 
+0.01 
+0.02 
+0.02 
+0.02 
+0.01 
+0.02 
+0.01 
+0.02 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 

I,  II 
n,  I 
I,  II 

n,  I 

I.  II 

II,  I 

I,  II 

II,  I 

i,n 
1,11 

+0.02 
+0.02 
+0.03 
+0.01 
-0.01 
—0.01 

+0. 00  ! 

+0.01 
+0.01     1 
+0.00    1 

320  GEOLOGY  OF  THE  OOMSTOCK  LODE. 

Steel  plugs  are  therefore  not  greatly  inferior  to  those  of  copper  or  zinc 
in  cases  whei'e  a  few  hundredths  of  a  volt  are  believed  to  be  of  minor  im- 
jDortance;  whereas,  on  the  other  hand,  their  use  for  the  purpose  in  view  is 
attended  with  much  convenience.  It  was  found,  however,  that  great  care 
had  to  be  taken  in  keeping  them  bright,  as  otherwise  the  electrical  difference 
between  the  gads  themselves  was  apt  to  rise  to  many  times  the  value  given 
above.  It  was  also  necessary  to  maintain  a  thorough  contact  between  the 
ends  of  the  metallic  circuit  and  the  gads. 

Great  difficulty  was  encountered  in  avoiding  leaks  in  the  copper  wire 
connecting  the  plugs  with  the  galvanometer.  At  first  wire  covered  with  a 
double  thickness  of  cotton  and  waxed  was  employed,  but  proved  to  be 
wholly  inadequate.  Even  gutta-percha  wire  scarcely  offered  as  complete 
an  insulation  as  was  desired,  in  the  hot  and  damp  atmosphere  of  the  CoM- 
STOCK,  when  laid  in  long  lines  without  special  precautions.  After  testing  a 
number  of  devices,  it  was  finally  found  sufficient  to  suspend  the  wire  from 
silk  or  waxed  cotton  threads,  care  being  taken  to  prevent  it  from  anywhere 
touching  either  rock  or  timbers.  This  plan  of  swinging  the  line  was  adhered 
to  throughout,  in  spite  of  the  loss  of  time  frequently  occasioned  thereby.  In 
short,  the  rule  was  finally  adopted  of  arranging  all  the  connections  just  as 
though  the  experiments  contemplated  were  to  be  made  with  frictional  elec- 
tricity. 

The  galvanometer  used  in  these  experiments  was  an  ordinary  instrument 
with  an  astatic  needle,  capable  of  measuring  intensities  as  small  as  0.0001 
in  "Weber's  electromagnetic  scale  {mg.  mm.  sec.)  with  certainty.  Readings 
were  made  directly,  the  needle  swinging  over  a  graduated  arc. 

For  the  measurement  of  electromotive  forces  a  method  of  compensation 
was  first  employed.  But  in  the  course  of  the  investigation  it  was  found 
absolutely  necessary  to  abandon  all  complications  and  to  reduce  the  method 
of  research  to  the  utmost  simplicity.  This  will  be  evident  to  the  reader 
when  he  remembers  that  the  heat  of  the  mines  is  such  as  to  cause  profuse 
perspiration,  and  thus  seriously  interfere  with  manipulation;  that  it  was 
desirable  to  make  the  first  observations  near  or  on  the  vein — hence  in  the 
busiest   part  of  the   mine — so  that  expeditious  operation  was  extremely 


ELEOTEIOAL  ACTIVITY  OF  ORE  BODIES.  321 

important;  that,  finally,  the  time  during  which  exposure  to  high  tempera-' 
tures  can  be  endured  with  safety  is  itself  necessarily  limited.  A  simple 
method,  analogous  to  one  of  consecutive  substitution  of  two  elements  in 
the  same  circuit  of  large  resistance,  was  therefore  adopted.  If  e  and  E  denote 
the  lode  electromotive  force  and  the  electromotive  force  of  a  normal  element, 
respectively,  i  and  I  the  intensities  due  to  the  action  of  e  and  E^e  in  the 
same  circuit,  we  shall  have,  approximately,* 

=  ^,  or  eznE 


E^e~r         ~      I^i 

Intensities  were  measured  by  the  aid  of  the  galvanometer  above  de- 
scribed, the  instrument  having  been  carefully  calibrated  at  the  outstart — an 
operation  which  was  frequently  repeated  during  the  course  of  the  experi- 
ments, i  and  I  could  both  be  determined  in  the  same  circuit  without 
inserting  auxiliary  resistances. 

Results. — By  way  of  example,  some  of  the  results  obtained  in  the  mines  of 
the  CoMSTOCK  will  now  be  cited.  The  plan  has  been  indicated  in  a  forego- 
ing paragraph  (page  3 1 6-7).  It  will  be  remembered  that  a  permanent  contact 
placed  conveniently  in  one  end  of  the  network  of  drifts,  is  successively  con- 
nected with  points  in  positions  of  sufficient  interest  to  justify  measurement. 
In  the  tables,  unless  otherwise  stated,  P.  C.  is  to  be  understood  as  coinciding 
with  point  I.  The  second  column  contains  the  distance,  in  feet,  of  the 
points  tapped  below  the  level  of  the  mouth  of  the  shaft  as  a  datum.  "  Dis- 
tance" and  "bearing"  refer  to  the  imaginary  lines  connecting  P.  C.  (I) 
with  the  remaining  points  of  the  series.  An  exception  is,  however,  made  in 
Table  VI.,  where  the  data  contained  in  corresponding  columns  give  the 
horizontal  distance  and  bearing  of  the  lines  joining  consecutive  points  e, 
the  lode  electromotive  force,  is  expressed  in  volts,  and  is  taken  as  positive 
when  it  acts  in  the  direction  P.  C. >     Earth >-  T.  C. 

'Approximately,  because,  in  the  case  when  the  lode  electromotive  force  acts  alone,  we  have  not  a 
true  circuit,  in  the  ordinary  sense.  Between  the  holes,  both  in  the  earth  and  in  the  wire,  the  direction 
of  the  current  is  the  same.  But  since  the  resistance  of  the  rock,  passing  from  the  hole  into  the  earth, 
diminishes  rapidly  (.see  page  '^^9),  the  former  may  be  considered,  with  a  degree  of  accuracy  sufficient  for 
the  purpose,  as  acting  through  the  same  resistance  as  does  the  normal  element,  subsequently  inserted. 

21  0  L 


322 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  IV. — Experiments  made  in  the  Ophir  mine. 

[Steel  gads.] 


No. 

Leyel. 

Points. 

Distance. 

Bearing. 

e. 

Remarlcs. 

1 
2 
3 
4 
5 
« 
1 
8 
9 
10 

Feet. 
2,000 

2,000 

2,000 

2,000 

2,000 

2,300 

2,300 

2.300 

2,300 

2,300 

I 

n 
ni 
rv 

III,  IV 

I 
n 
m 

IV 
V 

Feet. 
0 

116 

170 

415 

260 

0 

230 

370 

415 

470 

±0.00 
+0.02 
+0.01 
+0.02 
+0.01 

In  quartz  seam;  barren. 

In  clay  seam. 

In  quartz  seam ;  old  stope;  low-grade  ore. 

In  quartz  seam ;  new  stope ;  ore. 

In  clay  seam. 

In  small  quartz  seam ;  barren. 

In  small  quartz  seam ;  low-grade  ore. 

Do. 
In  quartzose  clay. 

S.  55°  E. 
S.  20°  E. 
S.  42°  E. 
S.  55°  E. 

N. 19°  E. 
N.  19°  E. 
N. 29°  E. 
N.  38°  E. 

+0.04 
+  0.01 
+0.05 
+0.02 

Table  V. Experiments  in  the  Consolidated  Virginia  and  California  mines. 

[Steel  gads.] 


1 

1,750 

I 

0 

+0.00 

2 

1,760 

n 

20 

e  *  °  i; 

HO      o 

+  0.09 

All  points  in  the  vein ;  ledge  very  broad ; 

3 

1,750 

in 

60 

Points 
verti 
above 
anotb 

+0.01 

low-grade  ore  in  quartz  gangue. 

4 

1,750 

rv 

100 

+0.08 

Table  YI. — Experiments  in  the  Ophir  and  Mexican  mines, 

[Copper  terminals.] 


1 

2 
3 

2,000 
2,300 
2,300 

+0.00 
+0  02 
+0.03 

In  small  quartz  seam  ;  barren. 
Do. 

in 

100 

S.190W. 

Do. 

4 

2,300 

rv 

100 

S.19°W. 

+0.04 

Do. 

5 

2,300 

V 

100 

S.19°W. 

+0.04 

In  large  quartz  seam  ;  low-grade  ore. 

6 

2,300 

VI 

80 

N. 29°  E. 

+0.03 

Do. 

7 

2,300 

vn 

85 

X.  38°  E. 

+0.04 

In  quartzose  clay. 

Discussion. — From  a  comparison  of  Tables  I.  and  II.  vrith  Tables  IV.,  V., 
and  VI.  it  appears  at  once  that  the  electromotive  forces  due  purely  to  chem- 
ical difference  and  polarization  of  the  terminals  are  of  the  same  order  as  the 
data  expressing  the  electric  activity  of  the  Lode.  The  latter  therefore  can 
serve  no  other  purpose  than  that  of  aflPording  information  as  to  the  magni- 
tude of  the  forces  to  be  determined.  To  assure  myself  as  to  the  certainty 
of  this  conclusion,  I  made  a  measurement  of  the  electromotive  force  (e)  ob- 
tained by  using  terminals  of  copper  and  zinc  conjointly,  and  found,  as  a  mean 

of  three  experiments, 

£=0.82. 

In   consequence  of  polarization,  the  current  speedily  diminished  in 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES.  323 

strength,  so  that  all  the  phenomena  are  identical  with  those  which  would  be 
obtained  in  the  laboratory.  The  eflFect  of  polarization  in  distorting  the  true 
value  of  the  lode  currents  was  frequently  noticed,  but  it  would  be  super- 
fluous to  repeat  the  data  here. 

It  is  necessary  therefore,  in  order  to  obtain  satisfactory  results,  to  apply 
all  the  refinements  that  have  been  developed  for  problems  of  this  character. 
In  making  an  attempt  of  this  kind  in  the  mines  on  the  Comstock,  however, 
unusually  great  difficulties  would  be  encountered.  At  the  outstart,  the  fact 
that  the  observer  is  compelled  to  operate  with  wet  hands  must  be  considei'ed 
as  prejudicial  to  delicate  physical  experimentation.  But  there  is  a  more 
fundamental  difficulty.  It  will  be  remembered  that  the  ore  of  the  Comstock 
Lode  is  argentite  accompanied  by  gold,  probably  in  the  metallic  state,  finely 
disseminated  in  quartz.  At  the  time  of  the  experiments  the  mines  without 
exception  were  working  in  comparatively  barren  parts  of  the  vein,  so  that 
there  was  actually  more  mineral  possibly  possessing  electrical  properties 
(iron  pyrites,  etc.)  in  the  rocks  than  ore  in  the  ore-stopes.  In  such  a  case 
the  term  "  ore  body"  is  scarcely  applicable  at  all. 

The  result  of  circumstances  of  this  kind,  regarded  from  an  electrical 
point  of  view,  can  be  expressed  as  follows:  Either  there  will  be  no  electric 
action  at  all,  since  each  little  granule  of  ore  or  pyrite  may  be  considered  as 
surrounded  by  an  insulating  envelope  of  either  quartz  or  country  rock — 
whether  the  latter  be  considered  as  an  insulator  or  an  electrolyte  is  imma- 
terial— or  the  whole  District,  vein  and  rock,  is  to  be  regarded  as  the  field 
of  electric  action.  In  the  latter  case  an  equal  difficulty  occurs,  insomuch  as 
within  the  limited  space  open  to  the  observer  the  variation  of  potential  will 
be  inappreciable.  In  short,  from  the  peculiar  distribution  of  mineral  matter, 
electric  excitation  is  not  local  in  comparison  with  the  space  accessible  for 
experimentation. 

The  unusual  difficulty  with  which  a  correct  interpretation  of  results 
would  be  attended,  not  to  mention  the  loss  of  time  occasioned  by  the  fact 
that,  in  consequence  of  the  heat,  experimentation  cannot  be  long  continued, 
finally  induced  me  to  abandon  the  matter  at  the  Comstock  altogether — at 
least  until  definite  results  could  be  obtained  in  a  more  favorable  locality. 


324  GEOLOGY  OF  THE  COMSTOCK  LODE. 


EXPERIMENTS   MADE  AT  THE    RICHMOND    MINE,  EUREKA  DISTRICT, 

NEVADA. 

Opportunities  for  investigation. — III  determining  to  make  the  study  of  local  cur- 
rents a  part  of  the  work  to  be  done  under  his  charge,  Mr.  Becker*  had 
selected  both  the  Comstock  Lode  and  the  Eureka  district  as  available  local- 
ities, in  which  to  test  the  applicability  of  an  electrical  method  as  an  aid  to 
prospecting.  The  former  is  a  fissure  vein,  in  which  the  ore,  comparatively 
free  from  base  material,  is  scattered  irregularly  through  a  quartz  gangue. 
At  Ruby  Hill,  Eureka,  the  ore  is  principally  plumbic  carbonate  and  sul- 
phidp  and  oxide  of  iron — the  whole  containing  more  or  less  silver  and  gold — 
occurring,  moreover,  in  huge,  apparently  isolated  masses  in  limestone.  In 
most  of  the  cases  fissures  containing  vein  matter  and  connecting  the  cham- 
bers have  been  traced.  The  facilities  offered  for  the  prosecution  of  the 
investigation  by  the  Eureka  deposits  were  therefore,  to  all  appearances, 
unusually  great.  The  immense  ore  bodies  in  sight  were  furthermore  at  a 
mean  distance  of  not  more  than  400  feet  from  the  surface,  and  a  series  of 
electric  surveys  could  easily  be  carried  out  over,  through,  and  under  them. 
Finally,  it  appeared  not  at  all  improbable,  insomuch  as  the  ore  bodies  in 
places  extend  to  within  100  feet  from  the  surface,  and  are  in  fact  to  some 
extent  above  the  mean  surface  of  the  suri'ounding  country,^  that  local  elec- 
trical currents  might  actually  be  detected  on  the  surface  itself  In  consid- 
eration of  this  encouraging  prospect  due  pains  were  taken  to  work  up  all 
the  experimental  details  with  corresponding  care. 

Arrangement  of  terminals. — Above  all  thiugs  it  was  uecessary  to  devise  some 
method  of  obtaining  electric  contact  between  the  ends  of  the  metallic  cir- 
cuit and  the  rocks,  which  would  be  free  from  the  difficulties  met  with  in 
the  Comstock.  Metallic  plates,  etc.,  used  alone,  ai"e  objectionable  (see  page 
358) ;  but  it  is  clear  that  through  the  intei'vention  of  a  suitable  liquid,  effects 
of  polarization,  etc.,  can  be  avoided.     The  following  contrivance,  based  on 

'  C/.  First  Annual  Report  U.  S.  Geolog.  Survey,  p.  46,  1880. 

'Being  in  Ruby  Hill,  an  elevation  of  some  hundreds  of  feet  .ibove  Ihe  extensive  plain  partially 
HurroHiKling  it. 


ELECTRICAL  ACTIVITY  OF  ORB  BODIES. 


325 


the  well-known  fact  of  the  excellence  of  amalgamated  zinc  in  a  zinc  sul- 
phate solution,  for  the  purpose  in  question,  was  finally  adopted. 

Into  a  large  cork  a,'  Fig.  23  (longitudinal  section),  is  inserted  a  strip 
of  amalgamated  zinc,  e/,  about  one-half  inch 
broad,  to  the  top  of  which,  e,  a  gutta-percha- 
covered  copper  wire,  hik,  is  soldered.  Through- 
out the  greater  part  of  its  length  it  rests  against 
a  stick  of  wood,  cd,  cylindrical  above  at  c,  which 
end  is  to  be  thrust  through  a  perforation  in  the 
cork  a,  but  wedge-shaped  below,  d.  At  i  the 
wire  and  stick  are  firmly  tied  together.  A 
smaller  cork,  b,  secures  the  lower  end  of  both 
zinc  and  stick.  The  whole  is  surrounded  by  a 
piece  of  beef-gut,  gg  (free  from  salt),  tied  to  the 
corks  a  and  b,  as  shown  in  the  cut. 

Into  the  bag  (6  to  10  inches  long)  thus 
formed  is  poured  a  solution  of  zinc  sulphate, 
the  wooden  plug  I  being  for  this  purpose  re- 
moved and  a  small  funnel  inserted.  On  replac- 
ing the  plug  the  terminal  is  ready  for  use.  The 
object  of  the  stick  is  to  obviate  accidents  due 
to  breakage  of  the  zinc,  this  material  becoming 
very  brittle  by  amalgamation. 

Fig.  24  represents  the  terminal  in  place.  A  suitable  hole,  6  to  9  inches 
deep  and  1  to  1 J  inches  in  diameter,  is  drilled  into  the  rock  or  vein,  at  an 
angle  of  about  30°  with  the  vertical,  and  filled  with  a  solution  of  sodic  sul- 
phate or  water;  whereupon  the  bag  is  introduced  as  shown  in  the  figure. 
The  dotted  line  mn  indicates  the  level  of  the  outer  liquid.^  Solution  of  sodic 
sulphate  was  at  first  used,  because  it  increases  the  conductivity  and  is  not 
acted  upon  appreciably  by  the  rock  (limestone).  It  was  found,  however, 
that  ordinary  water,  which  had  previously  been  placed  in  contact  with  zinc 
for  some  time,  so  as  to  precipitate  all  dissolved  matter  which  might  act  upon 

'  1  to  li  inches  in  diameter. 

'  The  siilutioa  poured  into  the  hole  will  be  referred  to  throughout  this  description  as  the  "  outer 
liquid." 


Fig.  23. — Terminal,  longitudinal 
section. 


326 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


it,  was  preferable  (see  page  357).  When  not  in  use  the  bags  were  kept 
in  a  glass  vessel  containing  a  zinc  sulphate  solution;  during  the  obser- 
vations, however,  they  were  transported  from  place  to  place  in  jars  con- 
taining water.^ 

The  electromotive  force  between  two  similar  bags  placed  in  the  same 

external  liquid  was  seldom 
found  to  be  greater  than  0.005 
volt,  usually  much  less,  and  tol- 
erably constant  (see  page  362); 
whereas  the  electromotive  force 
of  polarization,  due  to  the  ac- 
tion of  a  Daniell  under  circum- 
stances actually  met  with  in  the 
mines,  a  number  of  data  being 
in  hand,  was  in  no  case  as  large 
as  0.001  volt  and  in  the  experi- 
ments cited  falls  below  this 
limit.  For  comparison  the  bags 
in  a  particular  instance  were 
filled  with  water  instead  of  zinc 
sulphate,  when  an  electromo- 
tive force  of  polarization  of 
0.020  volt  was  obtained. 

Fig.  24.-TerminaI  in  position.  Wir=.  — Gutta-percha-COV- 

ered  wire  No.  19,  of  excellent  quahty  (Tillotson  &  Co.,  New  York),  was 
used  almost  exclusively,  the  whole  circuit  nevertheless  being  suspended  in 
air  from  threads,  as  in  the  Comstock.  In  the  long  circuit  on  the  600-foot 
level  it  was  necessary,  however,  to  employ  cotton-covered  wire  for  part 
of  the  line,  the  supply  of  the  other  being  insufficient.  This  could  be 
done  without  disadvantage,  as  follows:  A  hollow  cyHnder  of  gutta-percha, 
stripped  from  the  end  of  a  wire  covered  with  this  substance,  was  bent  in 
the  form  of  a  loop,  Fig.  25,  and  kept  bent  by  a  thread  passed  through  its 

'  It  was  desirable  during  the  observation  to  have  the  outside  of  the  bag  as  free  irom  zinc  sulphate 
solution  as  possible. 


ELEOTRIOAL  ACTIVITY  OF  ORE  BODIES.  327 

interior  and  tied.  The  cotton-covered  wire  used  (a  b  in  figure)  was  passed 
through  this  loop,  suspended  by  the  other  end  of  the  thread. 

A  case  in  which  gutta-percha-covered  wire  trailed  on  the  ground  a 
distance  of  about  1,000  feet,  was  made  the  subject  of  measurement.  A  leak 
was  quite  perceptible;  the  insulation  offered,  however, 
was  about  1,000,000  ohms. 

In  extending  the  line  from  point  to  point,  accord- 
ing to  Reich's  very  convenient  plan,  the  wire  is  wrapped 
on  a  light  wooden  reel,  but  in  such  a  way  that  the 
inner  end  also  remains  accessible.  The  outer  end  being 
in  connection  with  the  measuring  apparatus,  enough 
wire  is  uncoiled  to  reach  the  desired  hole,  and  a  con-  fig.  25.— Suspension. 
nection  (contact-bag)  between  this  and  the  inner  end  of  the  wire  is  then 
made.  In  the  damp  atmosphere  the  reel  soon  became  saturated  with 
moisture,  and,  in  spite  of  the  insulation  of  the  wire,  care  had  to  be  taken  to 
insulate  the  former  also. 

Galvanometer. — For  tho  measuremcut  of  intensity  I  was  fortunate  in  secur- 
ing a  magnificent  instrument,  made  for  me  after  the  Wiedemann  pattern,  by 
Mr.  "Wm.  Grunow,  of  New  York.  This  instrument  is  exceedingly  conve- 
nient for  the  purpose,  as  by  an  adjustment  of  the  coils  the  sensitiveness  can 
be  varied  over  a  very  wide  range.     Readings  were  made  with  telescope, 

20 
mirror,  and  scale.     In  the  adjustment  adopted  currents  as  small  as  —^ 

webers  could  be  detected  with  certainty. 

Measurement    of  electromotive    force. The     simple     method     of     COUSeCUtivC     Sub- 

stitution  for  the  measurement  of  electromoti v^e  forces  (  ezzE         .  \ — i 

somuch  as  while  there  were  no  reasons  for  abandoning  it  there  were  a  great 
many  in  its  favor — was  adopted  here  as  on  the  Comstock.  The  coils  of 
Grunow's  galvanometer  could  easily  be  so  placed  as  to  enable  the  observer  to 
measure  with  sufficient  accuracy  both  the  lode  current  and  that  due  to  the 
latter  and  the  normal  electromotive  force  conjointly,  without  making  any 
change  at  the  instrument  or  inserting  auxiliary  resistances.  By  means  of  an 
inclosed  mercury  commutator  the  current  in  the  galvanometer  could  be 


ui- 


328  GEOLOGY  OF  THE  COMSTOGK  LODE. 

reversed  and  the  deflection  thus  doubled.  All  intensities  (i  and  I)  were 
determined  as  a  mean  of  five  consecutive  commutations — not  that  it  was 
desirable  or  necessary  to  increase  the  accuracy  by  such  a  process,  but 
because  it  appeared  essential  not  to  hurry  the  measurements  and  to  test  the 
constancy  of  the  current  as  appeax-ingin  the  five  data  obtained.  Errors  from 
condensation  of  moisture  on  the  commutator  were  avoided  by  excluding  the 
latter  entirely  from  time  to  time,  the  measurements  being  made  by  simply 
connecting  the  wires  with  clamp-screws.^ 

As  a  matter  of  especial  importance  it  will  be  necessary  to  consider  a 
scheme  of  operations  by  which  discrepancies  due  to  extraneous  causes  can 
be  eliminated  as  comjiletely  as  possible.  In  the  experiments  the  following 
order  of  observations  was  adopted  and  rigidly  adhered  to  throughout: 

1.  Measurement  of  the  apparent  intensity  of  the  lode  current  (i'). 

2.  The  same,  with  the  terminals  exchanged  (i"). 

3.  Measurement  of  the  current  produced  by  the  normal  element  and 
lode  conjointly  (J). 

4.  With  the  battery  left  in  place  the  circuit  is  broken  at  the  temporary 
contact;  no  deflection  must  ensue  (E  supposed  to  be  acting  with  the  lode 
electromotive  force). 

If  a  mean  of  the  intensities  derived  from  the  first  and  second  opera- 
tions [i=:  J  (*"+*')]  ^6  taken,  the  intensity  of  the  current  (i)  due  to  the 
lode  only  will  be  obtained.  That  due  to  difi'erences  in  the  amalgamated 
zincs  is  thus  eliminated.  In  by  far  the  greater  number  of  experiments  three 
exchanges  were  made,  so  that  the  first  and  third  positions  of  the  terminals 
were  identical.     Analogously,  then, 


'  =  i 


(-+"^'-> 


The  fourth  operation  in  this  scheme  insures  the  perfect  insulation  of 
the  circuit  between  the  T.  C.  and  the  galvanometer.  The  part  between 
the  latter  and  P.  C. — the  two  being  always  placed  in  close  proximity,  this 

'The  commatator  used  wa8  made  of  wood  boiled  in  linseed  oil,  and  supported  on  three  conical 
feet  of  wood  boiled  in  wax  and  resin.  The  holes,  moreover,  were  coated  with  a  thick  layer  of  wax 
(see  page  354).  Whole  seta  of  observations  had  to  be  discarded  on  account  of  the  insufficient  insulation 
of  an  earlier  apparatus. 


ELECTRICAL  ACTIVITY  OF  OEE  BODIES. 


329 


partial  circuit,  moreover,  remaining  fixed — is  tested  once  for  all  before  com- 
mencing the  experiments. 

It  is  often  desirable,  before  inserting  the  Danieli,  to  determine  whether 
the  circuit  is  in  order  and  without  a  break.  This  may  be  easily  accom- 
plished by  touching  with  the  finger  a  copper  part  of  it,  so  that  a  secondary 

circuit,  T.  C,  wire,  galvanometer,  wire,  body,  earth,  T.  C,  or  P.  C,  wire 

body,  earth,  P.  C,  is  produced,  respectively.  The  electromotive  force  acting 
in  this  case  is  that  of  zinc-copper,  but  in  consequence  of  the  very  large 
resistance  of  the  finger  contact  the  current,  though  distinctly  perceptible, 
is  too  weak  to  produce  any  appreciable  polarization. 

In  spite  of  all  these  safeguards,  however,  a  close  inspection  of  the 
recorded  values  still  revealed  discrepancies  which  had  not  been  avoided. 
Accordingly  the  method  of  procedure  was  further  improved  by  the  follow- 
ing additions:  To  eliminate  as  much  as  possible  the  effect  due  to  the  terminal 
bags,  a  variation  was  introduced  by  which  the  results  from  different  bags 
could  be  compared.  Four  of  these.  A,  B,  C,  and  D,  were  generally  employed, 
which,  when  combined,  two  and  two,  in  the  manner  shown  in  the  diagram, 
gave  three  separate  and  distinct  values  for  the  lode  electromotive  force  e. 
The  electromotive  force  between  any  two  bags,  A  and  B,  is  represented  h\ 
AB,  between  A  and  C  by  ^|C,  etc. 


Holes. 

P.O. 



A 
A 
A 

T.C. 

Electromo- 
tive force. 

P.O. 

T.C. 

Electromo-   i 
live  force. 

P.O. 

T.C. 

Electromo- 
tive force. 

First  series  .  - . 
Second  series  . . 
Tliird  series  .  - . 

Ji 
0 
D 

e±A\B 
e±AiO 
e±AD 

B 
0 
D 

A 
A 

A 

e-fA\B 

e^A\C 
e^A  D 

A 
A 
A 

B 

C 
D 

etAB      \ 
e*AC       i 
e^zAD 

Original  positioo. 


First  exchange. 


Second  exi'hiinKc 


After  the  second  exchange,  the  bags  again  have  their  original  posi- 
tion with  reference  to  the  holes.  The  corresponding  measurements,  there- 
fore, check  one  another,  while  from  their  mean  any  linear  variation  of  their 
own  electromotive  force  is  eliminated.  Each  series  gives  a  value  for  e. 
With  this  method  of  triple  measurement  the  series  was  completed  by  deter- 
mining all  the  electromotive  forces  between  P.  C.  and  each  of  the  T.  C.'s, 
starting  with  the  one  nearest  P.  C.  and  ending  with  the  most  remote.  After 
this  the  whole  set  was  again  repeated,  starting,  however,  with  the  extreme 


330  GEOLOGY  OF  THE  COMSTOCK  LODE. 

T.  C.  and  finishing  with  the  one  nearest  P.  C.  The  two  sets,  therefore,  form 
a  symmetrical  series,  and  from  the  means  of  all  the  values  corresponding 
to  any  particular  T.  C.  any  change  which  may  have  taken  place  in  the  hole 
P.  C.  (see  page  360),  as  well  as  in  the  electromotive  force  of  the  Daniell, 
may  be  regarded  as  practically  eliminated.  A  comparison  of  the  two  sets, 
moreover,  affords  a  good  criterion  of  the  constancy  of  the  currents  as  well 
as  of  the  trustworthiness  of  the  results  obtained  in  general. 

Resistance. — Besides  the  electromotive  force,  the  resistance  of  the  differ- 
ent circuits  was  also  measured,  being  an  item  of  interest.  The  values  usu- 
ally ranged  between  2,000  and  3,000  ohms,  though  at  times  they  went  as 
high  as  20,000,  or  as  low  as  700  ohms.  Almost  the  whole  resistance  of  the 
circuit  is  encountered  by  the  current  in  passing  from  the  wire  into  the  rock, 
and  from  the  latter  back  again  into  the  former.  In  other  words,  the  resist- 
ance of  the  layers  of  rock  immediately  surrounding  F.  C.  and  T.  C.  is  so 
large  that  in  comparison  with  it  that  of  the  rest  of  the  circuit  (never  greater 
than  20  ohms)  can  be  completely  neglected  The  total  resistance  is,  there- 
fore, essentially  the  sum  of  two  terms,  corresponding  to  the  holes,  respect- 
ively. Suppose  now  that  in  a  circuit  P.  C.  (T.  C.)  these  partial  resistances 
are  w  and  r,  respectively;  in  a  circuit  P.  C.  (T.  C.)',  w  and  r',  respectively; 
if  it  is  found,  experimentally,  that 

w-\-r=:a,  ')  \ 

w-\- r'—  h,  \  ,  and  if  s  —  a-{-b-{- c,  then  {■)■'=:  ^  —  a, 

r+r'=c,)  ) 

\w^-  —  c. 

\        2 

These  points  have  been  described  in  considerable  detail,  being  of  such 
importance  that  without  them  the  results  reached  would  be  illusory.  I  was 
twice  obliged  to  discard  whole  sets  of  experiments  because  one  or  the  other 
of  the  disturbances  set  forth  had  found  their  way  into  the  results  in  the  most 
insidious  manner.  It  is  true  that  Fox  actually  used  uncovered  wire;  but 
it  must  be  remembered  that  the  currents  obtained  by  him  were  abnormally 
large.  Moreover,  I  am  convinced  that  the  currents  found  by  Fox,  when 
connecting  two  different  points  in  rock,  were  entirely  due  to,  and  that  those 


BLBCTRIOAL  ACTIVITY  OF  ORB  BODIES. 


331 


of  Reich  were  very  largely  distorted  by,  discrepancies  of  the  kind  discussed 
in  this  paragraph. 

Relative  position  of  the  ore  bodies. — Bcfore  procecding  furthcr,  it  will  be  neces- 
sary to  give  the  reader  a  general  idea  of  the  disposition  of  the  ore  bodies  of 
the  Richmond  mine.  It  will  be  convenient,  and  fully  sufficient  for  the 
present  purposes,  to  consider  them  with  reference  to  a  horizontal  and  a  ver- 
tical projection.  The  former  will  be  given  with  the  different  sets  of  obser- 
vations which  are  to  follow.  For  the  latter  I  am  indebted  to  Mr.  R.  Rickard, 
superintendent  of  the  Richmond  Mining  Company,  without  whose  cordial 
cooperation  it  would  have  been  impossible,  in  the  time  allotted,  to  carry  out 
these  experiments.  To  Mr.  Rickard  are  also  due  the  following  details  and 
sketch 


200' 


400- 


500' 


600' 


Fig.  26. — Vertical  section  through  ore  bodies. 

In  Fig.  26  the  horizontals  passing  across  the  diagram  represent  the 
levels  in  feet  below  the  shaft-mouth  as  a  datum.  Different  ore  bodies  are 
differently  shaded,  the  attached  numbers  depending  upon  the  date  of  their 
discovery.  The  sketch  is  intended  to  illustrate  the  relative  positions  of  the 
ore  bodies  one  to  another  only,  as  seen  from  the  extreme  north. 

Chamber  No.  11  begins  on  the  200-foot  level  and  continues  to  the  500- 
foot  level. 


332  GEOLOGY  OF  THE  COMSTOCK  LODE. 

No.  12  is  a  continuation  of  No.  11,  beginning  on  the  500-foot  level 
and  ending  70  feet  below  this  level. 

No.  16  commences  50  feet  above  and  runs  70  feet  below  the  200-foot 
level;  the  bottom  of  the  present  workings. 

No.  15  commences  on  the  300-foot  level  and  continues  to  the  500-foot 
level. 

No.  14  begins  50  feet  above  the  400-foot  level  and  continues  to  within 
50  feet  of  the  600-foot  level. 

No.  13  begins  at  the  500-foot  level  and  continues  50  feet  below  the 
600-foot  level. 

Chambers  Nos  13,  14,  and  15  are  all  connected  and  form  one  ore 
body.  No.  16  will  undoubtedly  connect  also  with  these  three,  so  that  in 
fact  Nos.  13,  14,  i5,  and  16  are  but  lobes  of  one  and  the  same  huge  deposit. 

The  greatest  horizontal  extent  of  these  bodies  is  between  the  400  and 
500-foot  levels,  the  plan  showing  the  following  dimensions: 

N.toS. 520  feet. 

E.to  W. .600  feet. 

No.  7  extends  from  the  400-foot  level  to  50  feet  below  this  level. 

No.  10  begins  20  feet  above  and  ends  50  feet  below  the  400-foot  level, 
and  is  exhausted.     No.  1 3  also  is  partially  exhausted. 

East  of  the  group  of  ore  bodies  of  the  Richmond  Company  are  those 
of  the  Eureka  Consolidated  Company,  which  are  also  of  unusually  large 
dimensions,  the  ore  being  the  same  in  every  respect. 

Experiments  on  the  500  and  400-foot  levels. — These  scries  of  measurements  were  made 
with  the  intention  of  observing  the  variation  of  potential  met  with  in  pass- 
ing through  the  ore  body,  the  line  of  electric  survey  beginning  and  termi- 
nating in  points  as  far  distant  from  it  as  was  practicable. 

The  plan  of  the  position  of  the  drifts  on  the  500  and  400-foot  levels 
relatively  to  the  ore  chambers,  so  far  as  is  necessary  for  the  present  purposes, 
is  given  in  Fig.  27,  on  a  scale  of  ~.  Starting  with  the  shaft  at  m,  the  drifts 
are  represented  by  broad  black  lines.  The  main  drift  on  the  400-foot  level, 
passing  from  a  point  between  VIII.  and  IX.  on  that  level  in  an  approxi- 
mately semicircular  path  toward  the  shaft,  has,  as  well  as  other  workings, 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


333 


been  partially  or  wholly  omitted.  Instead  of  giving  an  outline  of  the  hori- 
zontal projection  of  the  ore  bodies  themselves,  it  was  thought  preferable 
to  represent  rather  the  position  and  extent  of  the  actual  workings.  On  the 
map,  chamber  No.  11  is  designated  by  ab,  No.  12  by  CD,  Nos.  13  and  14 
by  rS,  and  No.  15  by  tg.     The  position  of  chambers  Nos.  7  and  10  is  only 


Fig.  27.— Plan  of  the  400'  and  .500'  levels.     Scale  rihu- 

indicated.     Smaller  patches  of  ore  also  occur  at  n,  between  the  .500  and 
600-foot  levels,  and  at  P,  above  and  below  the  500-foot  level. 

uv,  on  the  400-foot  level,  marks  the  position  of  a  line  of  contact  be- 
tween shale  and  limestone.     It  may  be  remarked  that  the  shale  of  the 


334  GEOLOGY  OF  THE  COMSTOCK  LODE. 

west  country  intersects  the  400-foot  level  on  a  line  approximately  parallel 
to  the  drift  between  P.  C.  and  No.  IV. 

Unfortunately,  local  circumstances  rendered  it  absolutely  impossible 
to  make  this  survey  in  a  single  continuous  series,  however  desirable  such  a 
method  of  procedure  would  have  been.  But  the  object  was  accomplished  indi- 
rectly by  selecting  a  permanent  contact  both  on  the  400  and  on  the  500-foot 
levels,  and  carrying  the  two  lines  of  measurement  onward  to  the  same  inter- 
mediate point.  The  differences  of  potential  thus  obtained  from  two  fixed 
points,  respectively,  can  then  be  converted  by  a  simple  method  of  reduction 
into  those  which  would  have  been  obtained  had  all  the  electromotive  forces 
been  measured  from  one  and  the  same  P.  C. 

On  the  500-foot  level  the  permanent  contact  was  placed  in  chamber 
No.  12,  in  calcareous  earth  stained  with  iron,  its  position  coinciding  nearly 
with  the  letter  G  in  the  plan  of  this  chamber  (Fig.  27,  C.  D.).  The  points 
selected  as  T.  C.^s  are  designated  on  the  map  by  small  circles,  to  which 
Roman  numerals  are  annexed,  and  extend  from  I.,  near  the  shaft  m  on  the  500- 
foot  level,  in  a  more  or  less  broken  line  to  XV.,  in  chamber  No.  15,  about 
30  feet  below  the  400-foot  level.  The  following  table  will  describe  them 
more  completely.  Column  2  in  Table  VII.  contains  the  points,  some  of  which, 
to  prevent  confusion,  were  omitted  on  the  map;  column  3,  the  depth  of  each 
below  the  mouth  of  the  shaft,  taken  as  zero.  "Distance"  refers  to  the  length 
of  the  lines  joining  consecutive  points  for  which  data  are  given.^  The 
figures  under  "bearing"  are  to  be  similarly  understood.  (S.  81°  W.  refers 
tothelinel.-IIL;  S.  26°W.,toIII.-V.;  N.67°  W.,toV.-IX.,etc.)  Itappeared 
unnecessary  to  give  more  than  the  bearings  of  the  main  lines  of  direction 
on  which  the  points  approximately  he.  The  figures  included  under  "re- 
sistance" are  the  means  of  two  determinations  of  this  quantity  made  for 
each  of  the  points.  They  express  the  sum  of  the  resistances  of  the  rock 
surrounding  P.  C.  and  the  T.  C.  specified.  The  original  results  were  always 
greater  than  those  made  at  a  subsequent  time;  this  from  the  fact  that  the 
rock  in  the  neighborhood  of  P.  G.  and  T.  G.  became,  during  the  progress  of 
the  experiments,  gradually  more  saturated  with  moisture. 

'  The  points  for  which  no  data  are  given  are  distributed  through  various  parts  of  chambers  12  and 
1.5,  in  positions  for  wliich  it  was  difficult  to  make  measorements. 


ELEOTEICAL  ACTIVITY  OJP  ORE  BODIES. 
Table  VII. 


335 


No. 


Points. 


P.C. 
I 
U 

in 

IV 
V 
VI 

vn 

VH' 

vm 
rx 

X 
XI 
XI' 
XII 

xni 

XIV 
XVI 
XVII 
XV 


Leyel. 


500 
500 
600 
500 
500 
500 
600 
600 
500 
500 
600 
500 
500 
490 
480 
460 
450 
460 
440 
430 


Dis- 
tance. I 


Bearing. 


Besist. 
ance. 


Feet. 


0 
84 


13 

47 


101 
101 

94 


Origin. 


S.  81°  W. 
S.  26=>  W. 


N.  67°  W. 


S.  80°  W. 


S.  37°  "W. 


Ohms. 


3620 
1480 
1560 
1660 
1670 

970 
2090 

850 
1520 
1590 
5660 
3720 
2240 
3590 
2620 
1990 
1140 
6260 

990 


Kemarks. 


\  Chamber  No.  12. 


Ferrnpnous,  calcareous  eartli,  in  chamber  12. 
Hard,  fissured  limestone. 
Limestone,  compact,  porous,  moist. 

Do. 

Do. 

Do. 
Limestone,  very  porous,  near  contact  of  chamber 
Ferruginous  earth. 
Red  ocher,  near  bunch  of  ore 

Ferruginous  earth , 

Pocket  of  lead  carbonate  ore  in  limestone, 
Hard,  impervious  limestone  and  calcspar. 
Hard,  solid  limestone. 

Ferruginous  earth,  with  galena 

Black  iron  ore,  loose  dry 

Ferruginous  earth,  with  galena 

Ferruginous  earth,  without  galena 

Large  breast  of  lead  carbonate  ore 

Ferruginous  earth,  very  dry 

Large  breast  of  lead  carbonate  ore 


The  results  of  the  measurements  of  electromotive  force  between  P.  C. 
in  chamber  12  and  the  consecutive  T.  C.'s  are  given  in  Table  VIII.  The 
general  method  of  obtaining  them  has  already  been  described  (see  page  329). 
Intensities  (i)  are  given  in  absolute  electromagnetic  units  (C.  G.  8.) ;  electro- 
motive forces  (e)  in  volts,  and  these  are  arbitrarily  considered  positive 
when  the  potential  of  T.  C.  is  the  greater,  or  when  the  lode  current  flows 

(wire} 

/earths 


T.  C. 


->p.  a 


It  will  be  remembered  that,  throughout,  four  terminal  bags.  A,  B,  C,  JD, 
were  used.  The  results  obtained  with  AB  are  given  in  Series  I.,  where, 
moreover,  i'  is  the  intensity  observed  with  the  bags  A  and  B  in  any  partic- 
ular position  (say  A  in  hole  P.  C,  and  B  in  T.  C.) ;  i"  the  intensity  observed 
when  the  bags  are  exchanged  (B  in  hole  P.  C,  and^  in  hole  T.  C);  finally, 
i'",  the  observed  intensity  when  the  bags  again  have  their  original  position. 
e  is  the  corrected  lode  electromotive  force  between  P.  C.  and  the  T.  C. 
specified.  Series  II.  contains  the  corresponding  results  with  the  bags  A  and 
C ;  Series  III.,  with  A  and  D. 

Finally,  Series  I.,  II.,  and  III.  were  obtained  in  surveying  from  point 


336 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


I.  to  XV.,  series  IV.,  V.,  and  VI.,  on  the  other  hand,  on  returning  from  XV. 
back  to  I 

In  these  experiments  a  solution  of  sodic  sulphate  was  used  as  an  outer 
liquid. 


Table  VIII. 

FIRST  SERIES. 


No. 

P.C. 
connected    v' X  10» 
with—    1 

I 

t"  X  10» 

v'"  X  10» 

«X10» 

P.C. 
No.   connected 
with— 

j 

t'xic 

i"  X  10« 

1 

1'"  X  10"  .    «  X  lO" 

1 
2 
3 

4 

1 
I         1    +    16 

n     1+41 
in            61 

-  10 

-  26 

_1_       flR 

+    13 

+  1 
+  1 

±     0 
±     0 
+    3 
+    2 

11    '        X 

X      n 

-     25 

4-     a 

7 

12     i       XI            -35 

—    6 

13  i       XI'           +41-46 

14  XII           -49+5 

15  !     XIII          -     25         -     25 

16  !      XIV      1-5         -     12 

17  '•      XVI      '     +     35          +10 

±     0 

TV               J_     ifi                     11 



-    8    ! 

6              V              -33+71 

6  i        VI            +89         -     66 

7  VII           +59         -     39 

8  VII'           +7          -     57 

9  i      Vin      1     +     35          -     41 
in      1         TV         1             117                     41 

-     30 

+  102 

—     6 

-     2 
+    3 

-     1 

±     0 
-  13 

19    !       XV 

+  112    j     +    95 

+  121 

+  1. 

1                   i 

1 

SECOND  SERIES. 


1 
2 
3 
4 
6 
6 
7 
8 
S 
10 


I 

n 
m 

IV 

V 
VI 
VII 
VTI' 

vin 

IX 


8 
30 
53 
39 
31 
71 
39 

0 

36 

120 


+ 


+ 


33 
46 
61 
33 
26 
57 
49 
38 


+  102 
+  43 
-      5 


-   1 

11 

±     0 

12 

-     2 

13 

-     1 

14 

+    2 

15 

+    3 

16 

+    2 

17 

1      -     3 

18 

-     1 

19 

-  13 

X 
XI 

XI' 

xn 
xm 

XIV 
XVI 

xvn 

XV 


+  2 

-  38 
+  49 

-  43 

-  16 

-  13 
+  53 

-  3 
+  82 


-  28 
+  25 

-  56 
+  12 

-  33 
±  0 

-  10 

-  2 
+  118 


+  uo 


7 
3 
1 
« 
6 
1 
2 

-     1 
+  11 


+ 


THIRD  SERIES. 


I 

n 
m 

IV 
V 
VI 

vn 

VII' 

VIII 

IX 


12 
38 
33 
33 
-  11 
+  97 
+    54 


+ 


13 
34 


31 
46 


-    34 


+    10    ■     - 
+    26 
-    05 


46 
46 


15 


-  11 
+  105 
+  67 
+      5 


+    2 
+    2 

-  2 

-  1 

-  11 


11 
12 
13 
14 
15 
16 
17 
18 
19 


X 

XI 
XI' 

xn 


-  5 

-  26 
+  44 

-  51 


xm 

-     20 

XIV 

-      8 

XVI 

+    36 

xvn 

-       0 

XV 

+  103 

-  25 

-  8 

-  49 

+  11 

-  28 

-  11 

+  10 

-  3 
+  U8 


8 
6 
0 
7 
6 
2 
3 

-     1 
+  11 


+ 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


.337 


Table  VIII— Continued. 

FOURTH  SERIES. 


No. 


P.  C 

connected 

with — 


I 
II 

ni 

IV 
V 
VI 

vri' 

VIII 


i'  X  io» 


+  t 

-  26 

+  8 

+  16 

+  57 

+  116 

+  33 

+  8 


i"  X  10' 


±  0 

+  25 

-  8 

-  16 
+  28 
+  66 
+  16 
+  15 


%"'  X  10« 


+     79 


ex  103 


No. 


P.  C 

connected 
■with- 


IX 
X 

XI 
XI' 

xn 
xnt 

XIV 


1'  X  io» 


81 
20 
31 

2 
25 
31 
28 


i"  X  10« 


57 
31 
16 
11 
15 
3D 
0 


i'l'  X  108 


ex  10' 


-  11 

-  14 

-  9 

-  1 

-  7 


FIFTH  SERIES. 


1 

2 
3 
4 
5 
6 
7 

I 
II 

m 

V 
VI 
VII' 

vrn 

+      7 
-     23 
+    16 
+    56 
+  105 
+    36 
+      0 

-  7 
+    20 

-  13 
+    23 
+    51 
+    13 
+    10 

±     0 
+     0 
±     0 
+    6 
+    7 
+    2 
+    1 

8 
9 
10 
11 
12 
13 
14 

IX 
X 
XI 
XI' 

xn 
xni 

XIV 

-  79 

-  21 

-  31 
+      2 

-  20 

-  38 

-  16 

-  61 

-  31 

-  18 

-  18 

-  18 

-  .20 

-  15 

-  11 

-  14 

-  9 

-  2 

-  7 

-  7 

-  3 

+    84 

SIXTH  SERIES. 


I 
II 
in 

V 
VI 
VTI' 

vni 


+  0 

-  25 

+  26 

+  56 

+  102 

+  34 

+  15 


+ 


+ 


+    51 
+    21 


+    90 


IX 
X 
XI 
XI' 

xn 
xm 

XIV 


72 
25 
26 

3 
26 
20 

5 


72 
31 
20 
23 
16 
25 
25 


11 
15 

8 
2 
7 
7 


400-foot  level. — The  permanent  contact  on  the  400-foot  level  was  placed  m  a 
ferruginous  clay  seam,  toward  the  southern  end  of  the  drift,  and  observa- 
tions were  made  in  a  northerly  direction  from  this  point.  The  temporary 
contacts  have  been  designated  on  the  map  (Fig.  27),  as  in  the  previous  case. 
Point  X.  of  the  present  survey  coincides  in  position  with  XV.  of  the  line  on 
the  600-foot  level.  The  following  table  (IX.),  in  which  full  statements  of 
the  position,  etc.,  of  the  points  are  contained,  will  be  intelligible  without  fur- 
ther description.  As  before,  the  bearing  of  the  main  linear  loci  only  have 
been  determined,  the  data  referring  to  the  lines  joining  the  consecutive 

points,  for  which  figures  are  given.     Resistances,  as  above,  are  mean  values 
22  c  L 


338 


GEOLOGY  OF  THE  OOMSTOCK  LODE. 


for   the   circuits   P.   C,   earth,  T.  C,  wire,  P.  C,  and  are    essentially  the 
resistances  of  the  layers  of  rock  surrounding  P.  C.  and  T.  C. 


Table  IX. 


No. 

Points. 

Level. 

Dis- 
tance. 

Bearing. 

Eesist- 
ance. 

Eemarks. 

1 
2 
3 
4 
5 
6 
7 
8 
9 
10 

n 

P.O. 
I 
II 

m 

IV 
V 
VI 

vu 

Vlil 
IX 

1     X 

Feet. 
400 

400 

400 

400 

400 

400 

400 

400 

400 

400 

430 

Feet. 
0 

100 

140 

139 

85 

37 

88 

94 

89 

68 

37 

Origin 

Ohms. 

2B90 
1040 
1820 
710 
2050 
1760 
2740 
1280 
1820 
1030 

Eed  clay  selvage. 

Black,  fissured  limestone,  dry. 

"White  calcareous  pulp,  very  moist. 

Gray  limestone,  compact,  dry. 

Shale,  very  moist. 

Gray,  fissured  limestone,  dry. 

Limestone,  compact. 

Do. 
Quartzite,  very  wet. 

Bunch  of  lead  carbonate  ore  in  limestone. 
Large  breast  of  lead  carbonate  ore,  chamber  15. 

1^.7°^. 

If.49oE. 

N.71°B. 

The  results  of  the  measurements  of  electromotive  forces  between  P.  C. 
and  I.-X.  are  contained  in  Table  X.  They  are  given  in  a  way  entirely 
analogous  to  that  adopted  for  the  500-foot  level,  and  no  further  explanation 
is  necessary.  Intensities  are  expressed  in  electromagnetic  units  (C  G.  S.), 
electromotive  forces  in  volts.     Water  was  used  as  an  outer  liquid. 

Table  X. 

FIRST  SERIES. 


No. 

P.C. 

connected 

with — 

i'  X  10' 

i"iy  10» 

i'"  X  10« 

ex  103 

No. 

P.C 

conn»Ci«d 

with— 

i'XlO* 

i"  X  10» 

i'"  X  10» 

exlO> 

1 
2> 
3 
4 
6 

I 

n 
m 

IV 
V 

+     22 

+    54 
—    49 
+      0 

+    il 

+    35 

+    11 

—  5 
+    10 

—  7 
+      1 

6 

7 

8 

9 

10 

VI 

vn 
vm 

IX 
X 

+  125 
+    56 
+     34 

+      i 
—    30 

+    93 
+    50 
+    43 

—  6 

—  56 

+  116 

+    19 
+     15 

+      4 
±       0 
—     4 

+    52         +61 

±      0 

+      2 

+      9 

—    21 

SECOND  SERIES. 


1 

I 

+    22 

+    39         +34 

+    10 

6 

Vt 

+  118 

+    88 

+  116 

+    19 

2> 
3 

n 
III 

vn 

+    60 
+    15 

+    45 
+    37 

+    15 
+      3 

+    65 

+    43 

+    63 

+    10 

8 

vm 

+      4 

4 

IV 

—    71 

—  130 

—    67 

—      7 

9 

IX 

—      4 

+      9 

—      6 

±       0 

5 

V 

+      8 

±       0 

+    13 

+      1 

10 

X 

—    63 

—    62 

—    37 

—      6 

1 

'  In  these  cases  three  consecutive  exchanges  of  the  terminals  were  made,  their  positions  in  Nos.  1  and  3  and  in  l^os.  2 
and  4  being  the  same. 


ELECTEICAL  ACTIVITY  OF  ORE  BODIES. 

Table  X — Continued. 

THIRD  SERIES. 


FOURTH  SERIES. 


339 


No. 

PC. 

connected 
with — 

i'X10« 

i"  X  10» 

i'"  X  10» 

ex  10' 

No. 

P.  C. 

connected 

with— 

i'  X  10« 

i"  X  10« 

i"'  X  10« 

eX10» 

1 
2> 
3 
4 
5 

I 

n 
ni 

TV 
V 

+    37 

—    9  1  — 
+    77 
—    73 
+    11 

+    30 

47  1  —  28 

+    50 

1  —  37 

+    11 

—  4 

+    10 

—  8 
+      1 

6 

7 

8 

9 

10 

VI 
Vii 

vni 

IX 
X 

+  138 
+    67 
+      4 
+      4 
—    13 

+    82 
+    37 
+    28 

—  6 

—  73 

+  140 

+    21 
+    15 
+      2 
±       0 

—      4 

+    30 

—  160 

—  7 

+    80 
—    52 
+    22 

+      6 
+    13 
—      0 

1 

I 

+    41 

+    22 

+    43 

+      9 

6 

VI 

+  140 

+    88 

+  142 

+    20 

2 

II 

—    76 

—  142 

-    88 

—    13 

7 

vu 

+    76 

+    58 

+    84 

+    19 

3 

in 

+    86 

+    56 

+    82 

+     13 

8 

vm 

+    62 

+    13 

•f    45 

+      4 

4' 

IV 

—118  1  - 

130  ]  —120 

1  —183 

—    10 

9 

IX 

±      0 

—    21 

—     2 

—      2 

5 

V 

+    35 

—    15 

+    24 

+       1 

10 

X 

—    30 

—    66 

—    24 

—      5 

FIFTH  SERIES. 


1 

I 

+    39 

+    28 

+    41 

+      9 

6 

VI 

4-  108 

+  108 

+  104 

+    19 

2 

n 

—    91 

—  130 

—  112 

—    13 

7 

vn 

+    60 

+     71 

+    62 

+    18 

3 

ni 

+    87 

+    62 

+    82 

+    14 

8 

vni 

+    19 

+    41 

+       4 

+      3 

4" 

IV 

—153  j  — 

45  1  —  4£ 

1  —220 

—      8 

9 

IX 

—    15 

—      9 

—    22 

—      2 

5 

V 

+      7 

+      6 

+      0 

+      1 

10 

X 

—    52 

—    34 

—    60 

—      6 

SIXTH  SERIES. 


1 

I 

+    45 

+    22 

+    49 

+      9 

6 

VI 

+  134 

+    78 

+  121 

+    18 

2 

n 

—    67 

—  130 

—    80 

—    12 

7 

VU 

+    80 

+    45 

+    76 

+    17 

3 

m 

+    82 

+     50 

+    82 

+     13 

8 

VIll 

+    58 

+      4 

+    45 

+      3 

4' 

rv 

-  91  1  - 

108  1  —112 

1  -168 

—      9 

9 

IX 

+      7 

—    28 

+      4 

-       2 

5 

V 

4-    24 

—    15 

+    21 

+       1 

10 

X 

-     13 

—    67 

—    11 

—     4 

^  In  these  cases  three  consecative  exchanges  of  the  terminals  were  made,  their  positions  in  Nos.  1  and  3  and  in  Nos.  2 
And  4  being  the  same. 

The  values  for  electromotive  force  contained  in  Tables  VIII.  and  X. 
are  now  to  be  referred  to  one  and  the  same  origin.  For  this  purpose  it 
will  be  convenient  to  select  a  point  having  an  extreme  position  Point  I., 
50()-foot  level,  is  of  this  kind.  As  there  is  no  means  of  assigning  an  abso- 
lute value  to  the  potential  of  this  point,  it  may  be  arbitrarily  called  zero,  in 
which  case  the  electromotive  force  between  it  and  any  succeeding  point  will 
be  identical  with  the  potential  of  the  latter.  In  the  following  table  (XI.) 
the  potentials  of  all  the  points  on  the  400  and  500-foot  levels  have  been 


340 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


calculated,  that  of  No.  I.  (500-foot  level)  being  zero.     The  values  obtained 

from  the  different  series  are  designated  by  indices  (e',  e",  e'",  e",  e^,  e^').     e^ 

is  the  mean  of  the  first  three,  62  of  the  last  three ;  and  e  the  mean  of  all  the 

series. 

Table  XL 


No. 

Points. 

Level. 

e'  X  105 

e"  X  103 

e>"  X  103 

ei  X  103 

e"  X  103 

e'X  103 

e"  X  10' 

CjX  103 

ex  103 

Feet. 

1 

I 

500 

+  1 

-  1 

+  0 

±  0 

+  1 

±  0 

—  1 

±  0 

+  0 

2 

n 

500 

+  1 

±  0 

±   0 

±  0 

±  0 

±  0 

±  0 

+  0 

±  0 

3 

ni 

500 

±  0 

—  2 

±  0 

+  1 

±  0 

±  0 

±  0* 

•      ±  0 

+  0 

4 

IV 

500 

±   0 

—  1 

±  0 

±  0 

±  0 

±  0 

+  0 

5 

V 

600 

+  3 

+  3 

+  3 

+  3 

+  5 

+  6 

+  6 

+  6 

+  5 

■< 

VI 

500 

+  2 

+  3 

+  2 

+  2 

+  8 

+  7 

+  7 

+  7 

+  4 

VII 

500 

+  2 

+  2 

+  2 
2 

+  2 

+  2 
±  0 

8 

VII' 

500 

-  2 

_   3 

—  2 

+  2 

+  2 

+  2 

+  2 

9 

VTTT 

500 

—  0 

—  1 

-  1 

—  1 

+  2 

+  1 

+  2 

+  2 

±  0 

10 

IX 

500 

—13 

—13 

—11 

-12 

-11 

—11 

-11 

-11 

—12 

11 

X 

600 

—  7 

-  7 

-8 

-  7 

-14 

—14 

-15 

—15 

—11 

12 
13 

XI 
XI' 

500 
490 

—  6 

±  0 

-  C 
±   0 

—  6 
±  0 

—  9 

-  1 

—  9 

-  2 

—  8 

—  2 

-  9 

2 

-  7 

-  1 

-  1 

14 

XTT 

■  480 

-  8 

-  6 

—  7 

—  7 

—  7 

-  7 

-  7 

—  7 

-  7 

15 

xin 

460 

—  6 

—  6 

—  6 

—  6 

-  8 

-7 

-  7 

—  7 

-  6 

10 

XIV 

450 

-  2 

—  1 

_  2 

—  2 

-  3 

—  3 

-  3 

-  3 

-  3 

17 

XVT 

450 

+  3 
1 

+  2 

_  1 

+  3 
_  1 

+  2 
—  1 

+  2 
—  1 

18 

XVII 

440 

19 

X'  or  XV 

430 

+  11 
+  15 

+  11 

+u 

+15 

+11 
+  15 

+11 

20 

IX  or  XVIII 

400 

+15 

+13 

+  13 

+13 

+  13 

+  14 

21 

VIII  or  nx 

400 

+  19 

+18 

+  17 

+  18 

+19 

+18 

+19 

+19 

+18 

22 

VCCor  XX 

400 

+30 

+30 

+30 

+  30 

+34 

+33 

+32 

+33 

+32 

23 

VI  or  XXI 

400 

+35 

+34 

+36 

+35 

+35 

+34 

+33 

+34 

+35 

24 

V  or  XXII 

400 

+16 

+16 

+16 

+16 

+  17 

+  16 

+  16 

+  16 

+16 

25 

IV  or  XXUI 

400 

+  8 

+  8 

+  7 

+  8 

+  5 

+  ^ 

+  6 

+  6 

+  7 

26 

ni  or  xxrv 

400 

+25 

+25 

+25 

+25 

+28 

+29 

+28 

+28 

+27 

27 

II  or  XXV 

400 

+  10 

+10 

+  11 

+10 

+  2 

+  2 

+  3 

+  2 

+  7 

28 

I  or  XXVI 

400 

+  26 

+26 

+26 

+26 

+24 

+25 

+25 

+25 

+25 

29 

P.  c.  or  xxvn 

400 

+15 

1 

iTo  facilitate  tlie  construction  of  Fig.  28,  current  numbers  have  been  given  to  the  points  on  ihc  400-foot  level.    Thenew 
numbers  are  given  with  the  original  ones. 

Table  XII.  has  been  prepared  to  show  the  character  of  e  as  a  function 
of  distance  (see  page  342).  In  it  e  has  the  same  signification  as  in  the 
preceding  table.  Under  distance,  however,  is  given  the  length  in  feet  of 
the  imaginary  line  joining  Point  I.  with  the  point  to  which  the  datum  refers. 
The  data  included  under  bearing  also  refer  to  this  line.  Current  numbers 
have  been  given  to  the  points  on  the  400-foot  level.  (See  "Points,"  Table 
XL) 


ELECTRICAL  ACTIVITY  OF  OEE  BODIES. 
Table  XII. 


341 


No. 

Points. 

LeveL 

Dis- 
tance 
&om  I. 

Bearing. 

eX10» 

No. 

Points. 

Level. 

Dis- 
tance 
from  I. 

Bearing. 

eXlOS 

Feet. 

Feet. 

1 

I 

500 

Origin 

+ 

0 

16 

XIV 

450 

635 

S.  72°  W. 

—    3 

2 

n 

500 

84 

S.  82°  W. 

± 

0 

17 

XVI 

460 

600 

S.  69°  W. 

+     2 

3 

in 

500 

123 

S.  81°  W. 

± 

0 

18 

XVII 

440 

640 

S.  75°  W. 

—     1 

4 

IV 

500 

168 

S.  63°  W. 

+ 

0 

19 

XV 

430 

700 

S.  72°  W. 

+  11 

5 

V 

500 

216 

S.  530  "W". 

+ 

5 

20 

xvm 

400 

735 

S.  72°  W. 

+  u 

6 

vr 

500 

228 

S.  54°  W. 

+ 

4 

21 

XIX 

400 

805 

S.  71°  W. 

+  18 

7 

vn. 

500 

268 

S.  60°  W. 

+ 

2 

22 

XX 

400 

890 

S.  73°  "W. 

+  32 

8 

VII' 

500 

275 

S.  64°  W. 

4; 

0 

23 

xxr 

400 

980 

S.  71°  W. 

+  35 

9 

vni 

500 

300 

S.  70°  W. 

± 

0 

24 

XXII 

400 

1066 

S.  71°  "W. 

+  16 

10 

IX 

500 

318 

S.  77°  W. 

_ 

12 

25 

XXTII 

400 

1108 

S.  70°  W. 

-1-    7 

11 

X 

500 

420 

S.  78°  W. 

— 

11 

26 

XXIV 

400 

1228 

S.  65°  W. 

-1-  27 

12 

XI 

500 

515 

S.  78°  W. 

— 

7 

27 

XXV 

400 

1184 

S.  69°  W. 

+     7 

13 

XI' 

490 

595 

S.  78°  W. 

— 

1 

28 

XXVI 

400 

1276 

S.  54°  W. 

+  25 

14 

xn 

480 

600 

S.  79°  W. 

_ 

7 

29 

XXVII 

400 

1332 

S.  51°  W. 

+  15 

15 

XTTT 

460 

610 

S.  79°  W. 

— 

6 

Discussion  of  tlie  results  obtained  on  the  400  and  500-foot  levels. FrOm  &  COmparisOIl  of  tllG 

resistances  of  circuits  between  diflferent  holes,  as  contained  in  Tables  VII. 
and  IX.,  we  find  that  in  cases  of  fissured,  of  tough  and  impervious,  or  of 
dry  rock  or  earth,  this  quantity  inclines  toward  a  maximum ;  whereas,  on 
the  other  hand,  wherever  the  material  is  porous  or  moist  minimal  values 
are  obtained.  It  is  to  be  remembered  that  under  ground,  from  the  exceed- 
ingly damp  atmosphere,  as  well  as  from  infiltration  of  water,  the  rock  form- 
ing the  walls  of  the  drifts  is  throughout  very  moist,  and  at  the  surfaces  of  the 
latter,  at  least,  nearly  saturated.  Hence  it  follows  that  the  conductivity  of 
the  rock  is  largely,  if  not  wholly,  due  to  the  presence  of  moisture  in  its 
pores,  and  is  therefore  electrolytic.  This  important  fact  will  be  repeatedly 
referred  to  hereafter. 

Intensities. — lu  Tablcs  VIII.  aud  X.  the  intensities  of  the  currents  ob- 
served in  the  different  circuits  have  been  very  fully  given,  both  because  the 
present  measurements  are  the  first  of  the  kind  made,  and  because  the  char- 
acter of  these  data  furnishes  an  important  criterion  of  the  validity  of  the 
results  subsequently  derived  from  them.  From  an  inspection  of  the  tables,  it 
is  moreover  obvious  that  an  exchange  of  terminals  in  measurements  of  this 
kind,  however  tedious  and  laborious  in  case  of  long  circuits,  is  indispen- 
sable.    The  intensities  i'  and  i'",  which  are  measured  with  the  bags  in  the 


342  GEOLOGY  OF  THE  COMSTOCK  LODE. 

same  position  relatively  to  the  holes,  are  usually  very  nearly  of  the  same 
value,  from  which  i"  generally  differs,  frequently  having  even  the  opposite 
sign. 

Potential. — Betwceu  the  values  of  e  for  the  first  three,  and  for  the  last 
three  series,  there  is  usually  a  good  agreement.  The  means  (ei  and  e^  of  these 
series,  however,  often  show  a  lack  of  accordance  which  is  greater  than 
was  expected.  The  discrepancies  occur  principally  in  the  results  obtained 
on  the  500-foot  level,  and  it  was  at  first  thought  that  they  were  largely  to  be 
referred  to  the  fact  that  a  solution  of  sodic  sulphate  was  used  as  an  outer 
liquid  In  the  holes.  In  No.  11,  Table  XI.,  for  instance,  this  liquid,  instead  of 
soaking  into  the  rock,  as  usual,  remained  in  the  hole,  gradually  becoming  con- 
centrated by  evaporation.  In  the  repetition  of  the  experiment,  therefore,  the 
exterior  liquids  in  P.  C.  and  X.  were  not  of  the  same  concentration,  so  that  a 
discrepancy  would  not  seem  remarkable.  Subsequent  experiments,  how- 
ever, hardly  corroborated  this  supposition.  Another  large  difi'erence  occurs 
in  the  case  of  No.  27  of  the  same  table;  but  for  this  hole  it  was  impossi- 
ble to  obtain  constant  results,  though  the  experiments  were  many  times 
repeated.     I  am  at  a  loss  to  account  for  this  fact. 

The  actual  relation  between  potential  and  distance  will,  of  course,  be 
exceedingly  complex,  and  it  would  be  little  short  of  a  waste  of  time  to 
endeavor  with  the  data  at  command  to  arrive  at  an  empirical  form  for 
this  function.  On  the  other  hand,  a  graphic  representation  of  the  change 
of  potential  due  to  a  corresponding  change  of  distance  is  certainly  desira- 
ble. Accordingly,  I  have  discarded  more  elaborate  mathematical  means  and 
have  represented  the  relation  in  question  by  the  following  simple  plan :  If  all 
points  on  the  400  and  500-foot  levels  be  joined  by  straight  lines  with  Point 
I.  on  the  500,  the  horizontal  projections  of  these  will  lie  within  a  sector 
whose  center  is  at  I.  and  whose  bounding  radii  subtend  an  angle  of  31° 
approximately.  It  should  be  noted  (Table  XII.)  that  on  passing  through 
the  oi'e  bodies  the  variation  of  bearing  is  much  smaller;  that  it  is  large 
both  for  points  near  I.,  where  the  actual  length  of  arc  subtended,  however,  is 
small,  and  for  points  on  the  400-foot  level,  where,  though  the  actual  length 
of  .subtended  arc  is  large,  as  all  points  are  remote  from  ore  a  smaller  change 
of  potential  may  be  expected.     Bearing  in  mind,  therefore,  that  the  object 


ELBCTEICAL  ACTIVITY  OP  OEE  BODIES. 


343 


is  merely  to  represent  in  a  systematic  way  the  potential  of  consecutive 
points,  a  curve  may  be  constructed  by  representing  the  linear  distance  of 
any  point  from  I.  as  abscissa,  the  corresponding  potential  as  ordinate.  In 
this  way  Fig.  28  was  obtained.  From  an  inspection  of  the  curve  it  appears 
that  the  ore  body  is  in  general  at  a  lower  potential  than  the  points  remote 


from  it. 


Kegion  of  ore  bodies. 


Country  rock. 


+50:105 


■AbJBIaBaBBkdPqaHB'BBaa    ' 
■■■■■■■BaHaHtjaHBaiBIIBlBri 


IBB 

■■HUB 


laarvjiBB— "s-'^Bawn 


IBBBBrvj 
■11        h 


BHPI 

■■■■■e  J 


I 


HBBBBBBBBBBIbMBB 
IBBBBBBBBBBBBadi 
IBBBBBB«BBBBflflBH 
^RBBBBBaBai 

nr 


0'  200'  400'  600'  800'  1000'  1200'  1400' 

Fig.  28. — Earth  potential  and  distance,  Richmond  mine,  400  and  500-foot  levels. 

Here  it  must  be  remarked  that  only  the  extreme  points  on  the  400-foot 
level  (XXVII.,  XXVI.,  etc.,)  can,  so  far  as  known,  be  considered  actually 
distant  from  ore.  In  the  vicinity  of  Points  I.,  II.,  etc.,  500-foot  level,  there 
are  not  only  the  streaks  of  ore,  n  and  p  (Fig.  27),  but  also  chambers  7  and  10, 
and  still  further  east  the  large  ore  bodies  of  the  Eureka  Consolidated  Mining 
Company.     This  has  been  indicated  by  the  dotted  lin.e  in  Fig.  28. 

The  variation  of  potential  is  irregular,  however — even  more  so  than, 
with  the  rough  method  of  delineation,  would  have  been  anticipated — and  its 
amount  is  small.  In  fact,  it  will  be  seen  that  certain  unavoidable  errors 
might  conspire  to  produce  an  almost  equivalent  change.  From  results  of 
such  a  magnitude,  in  short,  no  prediction  as  to  the  occurrence  of  ore  or 
electroactive  material  would  be  justified.  Not  to  mention  minor  matters, 
the  survey  described  suffers  from  a  serious  objection,  due  to  the  fact  that 
the  temporary  contact  in  progressing  from  I.  to  XXVII.  passed  through  a 
great  number  of  varieties  of  rock,  and  therefore  also,  probably  through  a 
great  variety  of  absorbed  liquids,  holding  more  or  less  saline  matter  in  solu- 
tion. In  such  a  case  the  electromotive  force  due  to  the  contact  of  these 
liquids  would  seem  to  come  into  play.  As  the  matter  will  again  be  dis- 
cussed (see  page  356)  I  will  add  here  only  that  electric  effects  thus  produced 
cannot,  a  priori,  be  regarded  as  negligible.     Furthermore,  the  preference 


344 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


given  to  Point  XV.,  in  using  it 
alone  as  a  basis  for  the  coordi- 
nation of  the  results  of  the  sur- 
veys on  the  500  and  400-foot 
levels,  is  to  be  criticised.     It 
was  intended    to  use   several 
consecutive  points  for  this  pur- 
pose; but  in   each  case  local 
interferences  prevented.     As  a 
whole,  however,  the  results  are 
-E    sufficiently  interesting  to  jus- 
3    tify  further  and  more  careful 
^    investigation. 

■5  Experiments  on  the  Soo-foot  level.    Results. 

"i    — This  series  of  measurements 

c 

>  1 1  was  made  with  the  intention  of 

'  '^_^  observing  the  variation  of  po- 

I  tential  encountered  in  passing 

1  across  the  ore  body,  without 
§  actually  entering  it.  Care  was 
^  also  taken  to  place  all  the 
S  points,  so  far  as  practicable, 
cj  in  rock  of  the  same  variety, 

2  and  to  remove  the  ends  of  the 
line  of  survey  as  far  from  the 
ore  body  as  possible. 

The  plan  of  the  position 
of  the  drifts  on  the  600-foot 
level,  relatively  to  the  ore- 
chambers,  is  given  in  Fig.  29. 
As  before,  the  points  tapped 
are  distinguished  by  small  cir- 
cles, to  which  Roman  numer- 
als are  annexed.     P.  C.  in  this 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


345 


case  coincides  with  Point  VIII.  and  is  in  porous  limestone.  The  great 
ore  bodies  have  been  lettered  as  in  Fig.  27.  Ore  is  also  found  at  G,  above 
and  below  the  600-foot  level,  and  at  n  above  it.  Z7  F  is  a  line  of  contact 
between  the  shale  of  the  west  country  and  limestone.  Table  XIII.  exhibits 
more  exactly  the  disposition,  etc.,  of  the  points.  It  will  be  intelligible  with- 
out further  explanation.     (Cf  Table  IX.,  page  338.) 

Table  XIII. 


No. 

Points. 

Dis- 
tance. 

Bearing. 

Resist- 
ance. 

Remarks. 

1 

I 

Feet. 
0 

Ohms. 
1580 

Shale,  moist. 

2 

n 

76 

S.  49°  E. 

2350 

Limestone. 

3 

in 

77 

S.  49°  E. 

1750 

Do. 

4 

IV 

65 

S.  49°  E. 

3110 

Do. 

5 

v 

75 

S.  49°  E. 

4015 

Do. 

6 

VI 

70 

S.  49°  E. 

4420 

Do. 

7 

VII 

87 

S.  62°  E. 

6300 

Do. 

8 

vm 

94 

S.  74°  E. 

Do. 

9 

IX 

85 

S.  74°  E. 

2480 

Do. 

10 

X 

71 

S.  74°  E. 

5150 

Do. 

11 

XI 

91 

S.  74°  E. 

3420 

Do. 

12 

xn 

80 

S.  74°  E. 

4170 

Limestone,  faintly  stained  witli  iron. 

13 

xin 

90 

S.  74°  E. 

3480 

Limestone. 

14 

XIV 

75 

S.  74°  E. 

3200 

Do. 

15 

XV 

78 

S.  74°  E. 

9490 

Limestone,  with  calcareous  spar. 

16 

XVI 

79 

S.  74°  E. 

15950 

Limestone,  hard,  impervious. 

17 

xvn 

127 

N.  72°  E. 

3440 

Limestone,  stained  with  iron. 

18 

XVIII 

118 

K.  85°  E. 

3380 

Limestone. 

19 

XIX 

72 

K. 85°  E. 

2525 

Pocket  of  fermginous  earth  in  limestone. 

20 

XX 

74 

S.  49°  E. 

3275 

Do. 

1     21 

XXI 

121 

S.  42°  E. 

4965 

Limestone. 

The  results  of  the  measurements  of  electromotive  forces  between  VIII. 
(P.  C.)  and  the  T.  C.^s  are  contained  in  Table  XIV.  The  nomenclature  being 
the  same  as  that  used  above,  the  meaning  of  the  data  will  be  at  once  appar- 
ent. As  before,  four  terminal  bags,  A,  B,  C,  and  D,  were  used.  Intensi- 
ties are  given  in  electromagnetic  units  (C.  G.  S.),  electromotive  forces  in 
volts ;  and  are  arbitrarily  considered  positive  when  the  potential  of  T.  C.  is 
greater  than    that   of  P.   C.  (Point  VIII.);  or  when  the    current   travels 

CpOTtJl) 

T.   C. >}     .     i >P.   C.     The    experiments  were    made   in   continuous 

(  wire  ) 

series,  starting  with  Point  I.  in  the  extreme  wes^  in  shale,  and  ending  with 
XXL,  near  the  shaft,  in  limestone.  Water,  which  had  previously  been 
kept  in  contact  with  zinc,  was  used  as  an  outer  liquid. 


346 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Table  XIV. 

FIRST  SERIES. 


SECOND  SERIES. 


1 

I 



172 

+ 

0 



193 

—  16 

11 

xn 

—    76 

—    93 

—    79 

—  33 

2 

n 

+ 

40 

— 

55 

+ 

38 

2 

12 

xni 

—  103 

—  122 

—  114 

—  40 

3 

m 

+ 

55 

— 

108 

+ 

53 

-    5 

13 

XIV 

—  160 

—  153 

-    152 

-  48 

4 

IV 

+ 

64 

+ 

71 

4- 

62 

+  18 

14 

XV 

—    56 

—     59 

—    60 

-  59 

5 

V 

+ 

47 

+ 

43 

+ 

50 

+  19 

15 

XVI 

—     SO 

—    55 

—    43 

—  90 

6 

TI 

+ 

17 

+ 

10 

+ 

19 

+     6 

10 

XVII 

—    !i8 

—  102 

-    96 

-  31 

7 

VII 

+ 

12 

+ 

5 

+ 

10 

+     5 

17 

xvin 

—  169 

-  182 

—  177 

—  55 

8 

IX 

+ 

9 

— 

9 

+ 

5 

±     0 

18 

XIX 

-    81 

—     72 

-    81 

-  18 

9 

X 

.  — 

24 

- 

28 

— 

21 

—  12 

19 

XX 

-    60 

—    69 

-    64 

—  17 

10 

XI 

— 

52 

— 

57 

— 

50 

—  16 

20 

XXI 

—    67 

—    64 

-    53 

—  30 

THIRD  SERIES. 


1 

I 



172 



3 



165 

—  17 

11 

xn 

-    72 

—  103 

-    79 

-  34 

2 

n 

+ 

33 

— 

38 

+ 

21 

—    2 

12 

xnr 

-  107 

—  134 

-  110 

—  41 

3 

m 

+ 

40 

— 

83 

+ 

36 

-    4 

13 

XIV 

—  160 

—  167 

-152 

—  50 

4 

rv 

+ 

65 

+ 

71 

+ 

02 

+  18 

14 

XV 

-    55 

—    57 

—    62 

—  59 

5 

V 

+ 

47 

+ 

34 

+ 

48 

+  17 

15 

XVI 

-    62 

—    57 

—    47 

—  96 

6 

VI 

+ 

19 

+ 

9 

+ 

17 

+     6 

16 

xvn 

—    91 

—  100 

—    95 

—  30 

7 

vn 

+ 

9 

+ 

5 

+ 

9 

+     4 

17 

xvni 

—  167 

—  182 

—  176 

—  54 

8 

IX 

+ 

10 

■  — 

19 

+ 

10 

—     1 

18 

XIX 

—    79 

-    71 

—    76 

-17 

9 

X 

- 

22 

— 

34 

— 

17 

-  12     , 

19 

XX 

-    64 

-    69 

—    65 

—  18 

10 

XI 

— 

48 

— 

71 

— 

48 

-  18 

1 

20 

XXI 

—    64 

—    62 

—    64 

—  30 

1 

FOURTH  SERIES. 


1 

I 



79 



98 



78 

-  11 

11 

XII 

-    83 

-     86 

-    81 

-  37 

2 

II 

- 

0 

- 

19 

- 

9 

-    3 

12 

xni  . 

-     96 

-  105 

-    95 

-  35 

3 

m 

- 

12 

- 

31 

- 

21 

-     4 

13 

XIV 

-  138 

-  146 

-  141 

-  46 

4 

rv 

+ 

48 

+ 

52 

+ 

59 

+  19 

14 

XV 

-    69 

-     71 

-     72 

-  59 

5 

V 

+ 

36 

+ 

38 

+ 

43 

+  15 

15 

XVI 

-     65 

-     69 

-     64 

-  95 

6 

VI 

+ 

14 

+ 

14 

+ 

7 

+     5 

16 

xvn 

-    72 

-    95 

-     69 

-  30 

7 

vn 

+ 

7 

+ 

T 

+ 

7 

+    4    1 

17 

xvm 

-  148 

-  143 

-  143 

-  52 

8 

IX 

± 

0 

- 

14 

• 

5 

-    2    1 

18 

XTX 

-     55 

-     78 

-    67 

-  18 

9 

X 

- 

22 

- 

21 

17 

-  11 

19 

XX 

-     60 

-    84 

-     60 

-  21 

10 

XI 

- 

41 

- 

47 

- 

43 

-  16 

20 

XXT 

-     52 

-    55 

-     53 

-  26 

ELECTEICAL  ACTIVITY  OF  ORE  BODIES. 


347 


Table  XIV— Continued. 
FIFTH  SERIES. 


No. 

P.C. 

connected 

with- 

i'  X  10» 

i"  X  10' 

i'"  X  10« 

ex  103 

No. 

P.C. 

connected 
with— 

i'  X  10' 

i"  X  10» 

i"i  X  10' 

«X103 

1 

I 

-    84 

-    83 

-     96 

-  11 

11 

XII 

-     84 

-     84 

—    86 

-  37 

2 

n 

-    12 

-    ID 

-     12 

-    3 

12 

XIII 

-  102 

-  103 

-  103 

-  36 

3 

in 

-    24 

-    16 

-    36 

-    4 

13 

XIV 

-  143 

-  141 

-  145 

-  46 

4 

IV 

+    48 

+     59 

+     65 

+  19 

14 

XV 

-    69 

-     72 

-     69 

-  59 

5 

V 

+     33 

+    43 

+     38 

+  15 

15 

XVI 

-    59 

-     71 

-     59 

-  93 

6 

VI 

+     10 

+     14 

+       7 

+    5 

16 

xvn 

-     74 

-    95  . 

-     65 

-  30 

7 

VII 

+       5 

+       9 

+       5 

+    4 

17 

xvm 

-  145 

-  145 

-  136 

-  52 

8 

IX 

-      9 

-      2 

-     12 

-    2 

18 

XIX 

-    60 

-    76 

-     64 

-  18 

9 

X 

-     22 

-     21 

-     21 

-  11 

19 

XX 

-    67 

-    76 

-    62 

-  20 

lu 

XI 

-    47 

-    43 

-    40 

-  16 

20 

XXI 

-    43 

-    55 

-     69 

-  27 

SIXTH 

SERIES. 

1 

I 

_ 

86 

_ 

90 

_ 

98 

-  12 

1   11 

xir 

-     64 

-    86 

-    84 

-  37 

2 

n 

- 

16 

- 

12 

- 

24 

-    3 

12 

xin 

-  110 

-  105 

-  105 

-  37 

3 

ni 

- 

36 

- 

21 

- 

38 

-    5 

13 

XIV 

-  138 

-  152 

-  138 

-  47 

4 

IV 

+ 

47 

+ 

62 

+ 

59 

+  20 

14 

XV 

-     72 

-    72 

-     72 

-  59 

5 

V 

+ 

31 

+ 

41 

+ 

36 

+  15 

15 

XVI 

-     00 

-    71 

-    64 

-  93 

6 

VI 

+ 

14 

+ 

7 

+ 

14 

+    5 

16 

xvn 

-     76 

-     93 

-     74 

-  31 

7 

YH 

+ 

7 

+ 

7 

+ 

9 

+    4 

17 

XVIII 

-  136 

-141 

-  141 

-  51 

8 

IX 

- 

24 

+ 

0 

- 

IC 

-    3 

18 

XTX 

-    65. 

-    78 

-     72 

-  19 

9 

X 

- 

19 

- 

22 

- 

21 

-  11 

19 

XX 

-    67 

-     76 

-     60 

-  19 

10 

XI 

- 

52 

- 

45 

- 

43 

-  17 

20 

XXT 

-    43 

-    50 

-     67 

-  28 

A  comparison  of  the  values  of  e  obtained  is  given  in  Table  XV.     The 
plan  is  analogous  to  the  above. 


Table  XV. 


No. 

Points. 

e'X10» 

e"  X  10' 

e"'x  10' 

ei  X  10» 

c"  X  10> 

e'XlOs 

e"X10' 

62X103 

«X10» 

1 

I 

-  16 

-  16 

-  17 

-  16 

-  11 

-  11 

-  12 

-   11 

-  14 

2 

II 

-    3 

-     2 

-     2 

-    2 

-    3 

-    3 

-     4 

-     3 

-    3 

3 

m 

-    3 

-    5 

-     4 

-     4 

-     4 

-    4 

-     6 

-    4 

-    4 

4 

IV 

+  17 

+  18 

+  18 

+  18 

+  19 

+  19 

+  20 

+  19 

+  18 

5 

V 

+  20 

+  19 

+  17 

+  18 

+  15 

+  15 

+  15 

4    15 

+  17 

6 

VT 

+     6 

+    6 

+    6 

+    6 

+    5 

+    5 

+    5 

+    5 

+    6 

7 

VII 

+    5 

+    5 

+    4 

+     5 

+    4 

+    4 

+    4 

+    4 

+    5 

8 

Yin 

±     0 

±     0 

±     0 

±     0 

±     0 

±     0 

±     0 

±     0 

±     0 

9 

IX 

±     0 

±     0 

-     1 

±     0 

-    2 

-     2 

-     3 

_     2 

-     1 

10 

X 

-  11 

-  12 

-  13 

-  12 

-  11 

-  11 

-   11 

-   11 

-  11 

11 

XI 

—  17 

-  17 

-  IS 

-  17 

-  17 

-  16 

-  17 

-17 

-  17 

12 

xn 

-  33 

-  33 

-  35 

-  34 

-  37 

37 

-  37 

-  37 

-  35 

13 

xni 

-  40 

-40 

-  41 

-40 

-  35 

-  36 

-  37 

-  36 

-  38 

14 

xrv 

-  48 

-48 

-  50 

-  49 

-  46 

-  46 

-  47 

-  46 

-  48 

15 

XV 

-  57 

-69 

-  69 

-  58 

-  59 

-  59 

-  59 

-  59 

-  59 

16 

XVI 

-  93 

-  90 

-  96 

-  93 

-  95 

-  93 

-  93 

-  94 

-  93 

17 

xvn 

-  30 

-  31 

-  30 

-  31 

-  30 

-  30 

-  31 

-  31 

-  31 

18 

XVIII 

-  57 

-  65 

-  55 

-   55 

-  .13 

-  52 

-  51 

-  52 

-  64 

19 

XIX 

-  17 

-  18 

-  17 

-   17 

-  18 

-  19 

-   19 

-  19 

-  18 

20 

XX 

-  18 

-  17 

-  18 

-  18 

-  21 

-  20 

-  19 

-  20 

-  19 

21 

XXI 

-  29 

-  30 

-  30 

-  30 

-27 

-  27 

-  28 

-  27 

-  29 

348 


GEOLOGY  OF  THE  COMSTOCK  LODE, 


Table  XVI.,  finally,  contains  the  data  necessary  for  the  approximate 
representation  of  earth-potential  as  a  function  of  distance.  By  arbitrarily 
assuming  the  potential  of  Point  VIII.  as  zero  the  final  means  in  Table  XV. 
are  identical  with  the  potential  of  the  points  to  which  the  data  refer.  The 
third  and  fourth  columns  of  Table  XVI.  contain  the  length  and  bearing  of 
the  imaginary  lines  joining  I.  with  the  succeeding  points. 

Table  XVI. 


Ifo. 

Points. 

Distance 
from  I. 

Bearing. 

ex  10" 

No. 

Points. 

Distance 
from  I. 

Bearing. 

eXVfi 

1 

I 

0 

Origin. 

-  14 

12 

XII 

850 

S.  6I0  E. 

-  35 

2 

n 

76 

S.  490  E. 

—    3 

13 

XIII 

935 

S.  630  E. 

-  38 

3 

m 

153 

S.  49°  E. 

-    4 

14 

XiV 

1010 

S.  640  E. 

-  48 

4 

VI 

230 

S.  49°  E. 

+  18 

15 

XV 

1080 

S.  640  E. 

-  59 

5 

V 

295 

S. 49°  E. 

+  17 

16 

XVI 

1160 

S.  650  E. 

-  93 

6 

TI 

365 

S:  49°  E. 

+    6 

17 

xvn 

1280 

S.  680  E. 

-  31 

7 

VII 

450 

S.  50°  E. 

+     5 

18 

xvni 

1380 

S.  70O  E. 

-  54 

8 

vin 

540 

S.  540  E. 

±     0 

19 

XIX 

1450 

S.  710  E. 

-  18 

9 

IX 

615 

S.  570  E. 

-    1 

20 

XX 

1510 

S.  70°  E. 

-  19 

10 

X 

685 

S.  580  E. 

-  11 

21 

XXI 

1630 

S.  70O  E. 

—  29 

11 

XI 

775 

S.  6OO  E. 

—  17 

Discussion. — From  the  results  in  Table  XIII.  for  the  resistance  of  dififer- 
ent  circuits  similar  conclusions  to  those  on  page  341  are  deducible.  Wherever 
the  structure  of  the  rock  and  coexisting  circumstances  are  favorable  to  the 
absorption  of  moisture,  there  also  minimal  values  for  this  quantity  are  found. 
Unusually  high  values  were  obtained  for  the  holes  XV.  and  XVI.  But  the 
rock  at  these  points  was  so  tough  and  tenacious  that  the  miners  complained 
of  the  slow  progress  made  in  drilling. 

Remarks  analogous  to  the  above  are  applicable  to  the  values  for  inten- 
sity on  this  level. 

The  results  for  earth-potential  in  Table  XV.  harmonize  much  better 
than  those  for  the  preceding  levels.  The  individual  values  in  the  two  series, 
as  well  as  the  means  of  the  series  themselves,  are  in  fair  accordance. 
This  might  be  ascribed  to  the  fact  that  the  holes  were  mostly  in  rock  of 
the  same  variety,  and  that  strong  salt  solutions  were  discarded  in  completing 
the  contact  between  the  terminal  bags  and  the  earth. 

By  a  method  of  procedure  similar  to  that  already  employed  the  relation 
between  potential  and  distance  may  be  represented  graphically.    It  will  also 


\ 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


349 


be  seen,  from  an  inspection  of  Table  XVI ,  that  the  considerations  involved 
in  constructing  Fig.  28  are  more  pertinent  in  this  case,  as  the  main  drift  itself 
is  more  nearly  linear.     Laying'  off  potential  as  or- 
dinate, distance  as  abscissa  (Table  XVI.),  Fig.  30  g      g      £ 
is  obtained.  5      5      i 

Both  Fig.  28  and  Fig.  30  demonstrate  the 
remarkable  result  that  the  region  of  ore  bodies 
coincides  with  a  region  of  low  potential.  This  is 
all  the  more  striking,  as  in  the  first  case,  taking 
in  general  a  northerly  course,  the  ore  bodies  are 
approached  from  barren  rock  (400-foot  level).  In 
the  second  the  course  of  survey,  while  passing 
toward  the  ore  region,  was  mainly  in  an  easterly 
direction.  The  two  lines  of  survey  may,  roughly 
speaking,  be  said  to  be  at  right  angles  to  each 
other.  In  one  case,  moreover,  the  sequence  of 
points  tapped  intersects  the  ore  bodies,  whereas  in  | 
the  other  it  remains  exterior  to  them  throughout  g 
its  whole  extent.  | 

a 

Comparing  the  results  of  the  two  surveys,  the 
indications  on  the  600-foot  level  are  found  to  be 
much  the  more  pronounced;  in  fact,  they  transcend 
values  which  can  be  accounted  for  as  an  aggregate  of 
incidental  errors.  It  may  be  remarked  here,  that  it 
is  a  very  impi-obable  chance  which  would  place  the 
region  of  greatest  electrical  disturbance  in  coinci- 
dence with  the  region  of  ore  bodies,  if  the  latter 
were  without  influence  in  producing  the  former. 
There  is  no  reason  apparent  why  the  part  of  the 
main  drift  on  the  600-foot  level,  between  Points  I. 
and  X.,  should  not  be  just  as  active  as  that  between 
Points  X.  and  XXL,  unless  it  be  that  these  points  lie  nearest  to  ore,  and 
consequently  that  we  are  here  rapidly  approaching  the  seat  of  an  electro- 
motive force. 


350 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Chambers  No.  14  and  No.  15  connected  electrically. In  the  SUrVej  Oil  the  400  and  500" 

foot  levels,  two  ore  bodies,  Nos.  12  and  15,  were  indirectly  connected, 
but  the  indications  obtained  were  much  smaller  than  was  anticipated.  It 
appeared  desirable  therefore  to  test  this  matter  still  more  carefully  by 
connecting  the  huge  ore-masses  in  chambers  No.  14  and  No.  15.  Ac- 
cordingly a  P.  C.  in  a  large  breast  of  ore  (lead  carbonate)  in  cham- 
ber No.  15  was  placed  successively  in  contact  with  three  diflferent  points, 
at  a  distance  of  about  100  feet  one  from  another,  in  chamber  No.  14.  Each 
of  these  was  also  in  ore,  the  first  in  lead  carbonate,  the  second  in  lead  car- 
bonate and  earthy  sulphide,  the  third  finally  in  a  mixture  of  carbonate, 
sulphide,  and  ferruginous  earth.  Table  XVII.  contains  the  results  of  the 
electrical  measurements.     A  single  set  of  observations,  with  one  exchange 

for  each,  was  made. 

Table  XVII. 

FIRST  SERIES. 


'So. 
1 

P.O. 
joined 
■with— 

i'  X  108 

i"  X  108 

«X10' 

1 

-     16 

-      3 

-    6 

2 

n 

-      8 

-     10 

-    7 

3 

m 

-     10 

-     13 

-    7 

SECOND  SERIES. 

1 

I 

-    18 

-      2 

-    6 

2 

n 

-      7 

-     10 

-    6 

3 

m 

-     11 

-     13 

-    7 

THIRD  SERIES. 

1 

I 

-     11 

-      8 

-    6 

2 

n 

-    11 

-     10 

-    7 

3 

in 

-    11 

-    13 

-     7 

In  considering  these  results,  it  is  strikingly  apparent  that  the  evidences 
of  electric  action  are  almost  altogether  absent.  It  is  true  that  in  all  proba- 
bility chambers  Nos.  14  and  15  are  but  parts  of  one  and  the  same  large 
ore-mass,  but  in  the  place  where  the  experiments  were  made  they  are  to 
some  extent,  at  least,  locally  disconnected.  The  results  lead  to  the  inference 
either  that  the  ore  of  both  chambers  is  remarkably  similar  in  character,  so 
as  to  present  no  appreciable  electric  difference,  or  that  it  is  here  without 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


351 


electrical  properties  altogether  (earthy),  the  field  of  electric  action  being 
confined  to  certain  definite  parts  of  the  ore-deposit.     (See  also  page  364.) 

Experiments  on  the  surface. — Encouraged  by  the  resiilts  on  the  600-foot  level, 
it  seemed  not  impossible  that  currents  might  also  be  observed  on  the  surface 
itself,  insomuch  as  the  ore  extends  in  places  to  within  100  feet  from  the  sur- 
face, while  vestiges  of  croppings,  etc.,  still  remain 

A  line  of  points  lying  in  general  in  a  north-and-south  direction,  and  at 
distances  of  about  1 00  feet  apart,  was  chosen,  the  object  being  to  extend 
the  electric  survey  from  shale  in  the  north,  free  from  ore,  over  Ruby  Hill 
and  the  large  ore  bodies  in  its  interior,  to  quartzite  in  the  south,  also  more 
or  less  free  from  ore.  It  was  hoped  that  in  this  way  a  passage  through  a 
field  of  electrical  activity  might  actually  be  made.  Unfortunately,  the  work 
was  interrupted  by  a  heavy  snow-storm  and  accompanying  frosts. 

P.  C.  was  placed  about  half  way  up  the  hill  in  compact  limestone. 
Point  I.  is  the  most  northerly  of  the  series,  and  remote  from  ore;  Point  IX. 
approximately  over  the  Richmond  ore  bodies.  The  results  are  contained 
in  the  following  table,  e  is  the  mean  of  a  single  triple  set.  The  potential 
of  P.  C.  (Point  VI.)  is  arbitrarily  put  equal  to  zero. 

Table  XVIII. 


No. 

Points. 

Eesistance. 

«X103 

Eemarks. 

1 

I 

17, 000 

-  20 

Bibris;  lowest  point. 

2 

II 

14, 000 

-  30 

Do. 

3 

HI 

13, 000 

-  30 

Do. 

4 

IV 

13,  000 

-  10 

Do. 

5 

T 

13,  000 

-  10 

Shale. 

fl 

VI 

dt     0 

Limestone!  (P.  C). 

7 

vn 

150,  000 

+  10 

Do. 

8 

vm 

40,  000 

+  20 

Limestone;  hif:Iiest  point. 

9 

IX 

20,  000 

+  40 

Do. 

10 

X 

25,  000 

+  50 

Do. 

In  the  table  the  unusually  high  values  for  the  resistances  of  the  cir- 
cuits, P.  C.  earth  T.  C,  are  a  striking  feature.  This  may  be  due  either  to 
the  compact  and  impervious  structure  of  the  rock  (the  drill  making  very 
slow  progress),  or,  as  the  experiments  were  made  in  the  early  spring,  to 
the  possibility  that  the  moisture  in  the  rock  was  still  frozen.  In  either 
case,  however,  the  supposition  that  the  conductivity  of  the  rocks  is  princi- 
pally due  to  the  presence  of  moisture  in  their  pores  receives  fresh  support. 


352  GEOLOGY  OF  THE  COMSTOCK  LODE. 

The  values  for  earth-potential  again  exhibit  a  marked  variation  in  pass- 
ing toward  the  ore-deposit.  But,  unlike  former  cases,  the  passage  fi'om  points 
remote  to  those  nearer  the  ore-region  is  one  from  lower  to  higher  potential. 
As  nothing  is  known  about  the  distribution  of  potential  with  reference  to 
ore  bodies,  this  is  not  to  be  regarded  as  at  variance  with  former  results. 
Not  overmuch  rehance,  however,  must  be  placed  on  the  values  of  e  in  this 
table.  They  were  obtained  under  unfavorable  circumstances,  and  not 
checked  as  in  the  former  cases. 

According  to  Matteuci,^  a  difference  of  potential  exists  between  points 
at  different  levels,  in  virtue  of  this  fact  alone.  "Ce  courant  est  ascendant 
dans  la  partie  mdtallique  du  circuit ;  son  intensity  augmente  h  mesure  que 
les  lignes  sont  plus  longues,  et  que  la  difference  de  niveau  entre  ces  ex- 
tremit(^s  est  plus  grande."  But  in  the  present  case  the  direction  of  the  cur- 
rent is  not  only  the  opposite  of  this,  but  the  electromotive  force  continues 
to  increase  even  in  greater  ratio  after  the  highest  point  of  the  series  has  been 
reached.  The  effects,  therefore,  are  not  such  as  Matteuci  observed.  The 
reader  is  further  referred  to  page  360. 

EEPETITION   OF   SOME   OF  THE  EXPERIMENTS  AFTER  AN  INTERVAL 
OF  ABOUT  ONE    HUNDRED  AND  THIRTY  DAYS. 

The  preceding  experiments  are  to  be  regarded  as  incomplete  in  two 
particulars.  In  the  first  place,  the  data  are  the  results  of  but  a  single 
method  of  measurement,  the  application  of  which  is  not  immediately  evident; 
in  the  second,  no  criterion  of  their  constancy  in  point  of  time  has  as  yet 
been  obtained.  The  additional  results  now  to  be  given  were  obtained  on  the 
600-foot  level  of  the  Richmond  mine,  all  of  the  former  holes  (points  tapped), 
with  the  single  exception  of  No.  I.,  being  used  over  again.  In  place  of 
the  latter,  this  having  become  inaccessible,  a  fresh  hole,  about  25  feet  to 
the  east  of  the  old  one,  but  also  in  shale,  was  drilled. 

The  experiments  were  made  after  an  interval  of  more  than  four  months 
from  the  time  at  which  the  original  data  were  obtained. 

Methods. — From  an  inspection  of  the  magnitude  of  the  electromotive 
forces  contained  in  the  foregoing  tables,  it  will  be  seen  that  they  fall  well 

'Ann.  de  Chim.  et  de  Pbys.,  (4),  T.  X.,  p.  148,  1867. 


ELECTRICAL  ACTIVITY  OP  ORE  BODIES, 


353 


within  the  scope  of  a  good  electrometer.  Such  an  instrument,  properly 
protected  against  the  moisture  of  the  underground  air,  would  have  been 
most  serviceable  for  the  purpose.  Unfortunately,  one  could  not  be  obtained 
in  time  for  the  work.     The  following  methods  were  therefore  resorted  to : 

In  the  first  place  the  greater  part  of  the  data  were  checked  by  the 
method  already  described.  This,  it  will  be  remembered,  was  chosen  because 
of  its  simplicity  and  the  comparative  ease  with  which  any  fault  in  the  con- 
nections could  be  ascertained. 

The  potential  of  the  same  holes  was  noW  measured  by  a  method 
in  which  the  electromotive  force  is  expressed  in  terms  of  the  increment  of 
the  reciprocal  of  intensity  of  current,  and  the  corresponding  increment  of 
the  resistance  of  the  circuit,  to  which  the  former  is  due.  In  order  to  vary 
the  resistance  at  pleasure  a  rheostat  was  introduced.  If  the  resistances  tVi 
and  W2  correspond  to  the  intensities  i^  and  is,  respectively, 


to,— 

W2 

1 

] ' 

h 

«2 

e  = 


where  e  is  the  electromotive  force  to  be  measured. 

Finally,  the  whole  of  the  experiments  formerly  made  on  the  600-foot 
level  were  again  repeated  by  a  zero 
method.  Here  great  care  had  to  be 
taken  to  effect  the  complete  insulation 
of  all  parts.  This  was  accomplished 
in  the  manner  previously  indicated, 
by  suspending  the  terminal  wires,  as 
well  as  all  the  connections,  from 
threads.  The  accompanying  diagram. 
Fig.  31,  will  show  how  this  was  done. 
A  and  B  are  clamp  screws,  suspended 
from  the  threads  a  and  d,  respectively, 
E  (rheostat)  is  the  large,  r  the  small 
resistance,  K  a  double  key,  C  a  com-  i*''**.  31.— Disposition  of  apparatus, 
mutator,  O  the  galvanoscope.     For  a  zero  current  in  the  latter  (the  effects 


354 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


due  to  tjie  normal  element  E  and  the  lode  electromotive  force  compensat- 
ing each  other  in  G),  approximately, 

r 


e  =  JE 


R-\-r' 


■■■■■I 

■■■■■I 


!■■«■■!■■■■ 

■■■■■  ■■«i|ilHB 
■■■■■  ■■■■■■■■■B 

■nliainiBH 

nldiflBBHBI — 

1Hb;s::i 

'§■■■■  ■ 

IS 


■■■■■ 
■■■■i 

sSap 

HI 


-  ■■■ 

■   HI 

i 


H 

■i 

■■■ 

■■■ 
■r 


■ 


IP>" 


li 

liiS 


I! 


■■■■■■ 


■■■■■■■nBB 


rssi 

Sing  ■■  ■ 

■«■■  ■■■■ 

ma  :b: 
b::: 


!■■■ 

8s;s 


The  resistance  r  was  wrapped  on  a  small  piece  of  wood  and  the  whole  sub- 
sequently boiled  in  paraffine.     The  body  of  the  key 
i       K,  and  that  of  the  commutator  C,  were  similarly  pre- 
pared, being  boiled  in  linseed  oil,  and  the  mercury 
cups  covered  internally  with  a  thick  coating  of  wax. 
I  -j  Moreover,  the  wires  of  both  in  passing  through  the 
I   wood  were  additionally  insulated  from  the  latter  by 
I   a   covering  of  gutta-percha;    the   ends    only  being 
i  X  uncovered  and  communicating  with  the  mercury  in 
s  the   cups.     In  consequence  of  these  precautions  it 
tS   was    found    that   this    comparatively   complicated 
I  §   method  could  be  em^jloyed  in  these  wet  drifts  with 
I   complete  success,  and  the  adjustments  having  once 
'§   been  made,  it  proved  to  be  nearly  as  expeditious  as 
i  I   either  of  the  other  methods. 
^^  As  the  result  obtained  is  derived  from  an  ex- 

I   pression  which  is  independent  of  the  resistance  of 
§  I   the  circuit,  the  method  could  be  used  with  advantage 
I   in  studying  the  manner  of  variation  of  potential  in 
I   passing,  as  it  were,  continuously  from  any  T.   C.  to 
*  ^  the  next.     But  the  actual  observations  will  be  more 
■3   appropriately  cited  in  connection  with  another  topic 
^   (see  page  361). 
"  n  It  will  be  remembered   that  in  the  former  ex- 

^   j^eriments  four  contact  bags  were  used  throughout, 
■■■■Eoii  I    iBii  which  were  so  combined  as  to  give  three  indepen- 

|SMSS'S5S  ■■■■■■■  „       dent  values  for  the  electromotive  force  to  be  meas- 
s       I       I  ured.     The    results    thus   obtained,  however,  being 

o  ^  "^ 

41        I       7  usually  so  nearly  identical,  it  was  thought  that  this 

precaution  might  safely  be  dispensed  witli.     Two  contact  bags  only,  there- 


■I 


nsssn  as 

■■■InlaB  ■■ 


■■■■■!■■ 
■■■■■■■! 

■■■■■■nE 


■■■■ 
■■■■ 
■■■■ 
■■■■ 


Hi 

as: 


ELECTRICAL  ACTIVITY  OP  OEE  BODIES. 


355 


fore,  were  employed.  In  all  other  respects,  however,  the  former  plan  (see 
page  328)  was  rigidly  adhered  to,  with  such  slight  variations,  of  course,  as 
the  different  methods  rendered  necessary. 

Results. — The  following  table,  containing  the  potential  of  the  consecu- 
tive points  on  the  6()0-foot  level — that  of  No.  VIII.  being  arbitrarily  put 
equal  to  zero,  as  before — will  be  intelligible  without  much  further  explana- 
tion. The  results  of  the  different  methods  are  arranged  in  parallel  columns, 
and  in  the  order  in  which  they  were  described.  For  the  sake  of  comparison 
those  obtained  in  the  foi'mer  survey  are  also  added,  and  a  final  column 
shows  the  difference  between  the  two. 

Table  XIX. 


e  X 10',  determined— 

No. 

Points. 

e  X  103, 
mean. 

e  X  103, 
old  value. 

6(e)  X 103. 

Remarks. 

By  old 
method. 

With 
rheostat. 

By  com- 
pensat. 

1 

I' 

+  ^ 
+  1 
+  1 

+  12 
+  15 
-  30 

+    7 
+     1 
+    1 

—  14 

New  and  old  holes 

2 
3 

n 
m 

-    3 
_    4 

-  4 

-  5 

do  not  coincide. 

4 

rv 

+  12 

+  18 
+  17 
+    6 

+    6 

5 

V 

+  15 

+    2 

6 

VI 

-  30 

+  36 

7 

Vll 

—    3 

_     4 

_    4 

+    5 
-     1 

+    9 

8 

IX 

-    1 

-    1 

-     1 

-     1 

±     0 

9 

X 

-  29 

-  25 

-  31 

-  28 

-  11 

+  17 

10 

XI 

-  25 

-  25 

-  25 

-  25 

-  17 

+    8 

11 

Xll 

-  13 

-  12 

-  14 

-  13 

-  35 

-  22 

12 

Xlli 

-  30 

-  23 

-  24 

-  29 

-  38 

-     9 

13 

XIV 

-  39 

-  39 

-  40 

-  39 

-  48 

-     9 

14 

XV 

-  41 

-  40 

-  47 

-  43 

-  59 

-  16 

15 

XVI 

-  72 

-  75 

-  76 

-  74 

-  93 

-  19 

16 

XVII 

-  15 

-  15 

-  23 

-  18 

-  31 

-  13 

17 

xvm 

-  48 

-  47 

-  50 

-  48 

-  54 

—    6 

18 

XIX 

-    8 

-  13 

-     7 

-    9 

-  18 

-     9 

19 

XX 

-  14 

-  13 

-  14 

-  14 

-  19 

-     5 

20 

XXI 

-  31 

-  32 

-  39 

-  34 

-  29 

+    5 

The  results  obtained  by  different  methods  present  throughout  a  fair 
agreement,  when  it  is  remembered  that  errors  amounting  to  a  few  thou- 
sandths of  a  volt  are  introduced  by  circumstances  beyond  the  observer's 
control.  Between  the  mean  of  the  new  and  the  mean  of  the  former  results 
thei'e  are  a  number  of  annoying  discrepancies.  In  part,  though  by  no  means 
wholly,  these  are  due  to  a  difference  in  the  values  of  the  standard  electi-o- 
motive  force  emiployed  in  the  two  cases.     With  the  knowledge  at  present 


356  GEOLOGY  OP  THE  COMSTOCK  LODE. 

available  it  would  be  of  little  use,  however,  to  attempt  to  assign  reasons  for 
tlie  remaining  variations.  A  matter  of  greater  importance  is  that  the  gen- 
eral character  of  the  curves,  as  derived  from  the  two  series  of  results,  is 
essentially  the  same.^ 

UNAVOIDABLE  EEEORS  AISTD  MISCELLANEOUS  CEITICISMS. 

Moisture  in  the  rocks. — Bv  far  the  most  seHous  difficulty  encountered  in  en- 
deavoring to  interpret  the  results  obtained,  is  that  due  to  the  difference  of 
potential  of  two  liquids  in  contact.  The  conductivity  of  rocks  is,  as  has 
been  seen,  largely,  if  not  wholly,  to  be  ascribed  to  the  presence  of  moisture 
in  their  pores.  This  moisture  unquestionably  holds  saline  matter  in  solu- 
tion. Moreover,  it  is  altogether  probable  that  the  solution  in  one  rock  of  a 
particular  structure  is  in  general  different  from  that  in  another  of  different 
structui'e  and  many  hundred  feet  distant  from  the  former,  even  if  the  com- 
position of  both  is  essentially  the  same.  In  tapping  two  points  at  some  dis- 
tance apart  by  the  aid  of  two  metals  (plates  or  gads)  supposed  identical  in 
every  respect,  two  members  of  the  continuous  sequence  of  solutions  con- 
tained in  the  rocks  are,  in  fact,  put  in  metallic  contact.  The  difference  of 
potential  thus  obtained  would  be  that  due  to  the  resultant  action  of  the 
series  of  liquids  included  between  the  points.  This  electromotive  force  is, 
however,  principally  dependent  on  the  extreme  members  of  the  sei-ies,  i.  e., 
those  at  the  points  tapped;  and  in  the  present  investigation  it  was  hoped 
that  the  discrepancy  thus  arising  might  be  very  largely  eliminated  by  put- 
ting the  same  liquid  in  both  holes,  and  by  exchanging  not  only  the  metallic 
terminals — amalgamated  zinc — but  also  the  terminal  solutions  (zinc  sulphate). 
Hence  the  "bag"  form  of  the  terminal. 

It  was  thought  not  superfluous  to  test  the  matter  with  the  aid  of  the 
contact  bags  themselves;  all  the  more  as  it  would  thus  appear  to  what 
extent  the  results  obtained  with  the  latter  are  trustworthy.  The  two  liquids, 
whose  electromotive  force  was  to  be  measured,  were  separated  from  one 
another  by  a  porous  septum  of  animal  membrane.  As  in  the  mines,  the 
terminal  bags  were  exchanged.  In  passing  them  out  of  the  first  liquid  into 
the  second,  care  was  taken  to  wipe  off  the  liquid  adhering  to  the  outside. 

'  Compare  Figs.  30  and  32. 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


357 


If  now,  e  be  the  electromotive  force  of  the  two  solutions  in  contact,  e  that 
due  to  the  difference  between  the  zincs  alone,  in  the  first  position  of  the  bags 
A  and  B  (A  in  water  and  B  in  the  liquid  to  be  tested),  the  apparent  force 
would  be 

in  the  second  position  of  the  bags  (B  in  water  and  A  in  the  Hquid  to  be 

tested), 

the  connections  themselves  remaining  unaltered.  A  mean  of  both  measure- 
ments gives  £;  half  the  difference,  e.     The  following  are  some  of  the  results: 


r  Both  bag8  in  water 

c  Bags  alternately  in  solntion  of  Na*  SO*  and  in  water. 

fHoth  bags  in  water 

i  Bags  alternately  in  salt  solution  and  in  water 

tBoth  bags  again  in  water , 

r  Both  bags  in  water 

c  Bags  alternately  in  Zn  SO'*  solution  and  in  water 


eXlQS 


1.0 
1.0 

2.8 
3.4 
3.4 


ex  10= 


0.4 
2.2 
0.6 

+  0.4 
-0.4 


It  will  be  seen  that  in  the  different  sets  s  is  fairly  constant.  The  value 
of  e  is  small,  as  anticipated,  notwithstanding  that  nearly  concentrated  solu- 
tions were  used.  In  the  case  of  zinc  sulphate  e  is  practically  zero,  as  it 
should  be,  the  bags  themselves  containing  this  solution.' 

The  following  table  contains  analogous  experiments  made  in  the  mines. 
Holes  IX.  and  X.,  600-foot  level,  were  put  in  contact.  Measurements  were 
made  by  a  zero  method: 


Water  in  both  IX.  and  X 

Water  in  X. ;  concentrjited  brine  in  IX. 
Concentrated  brine  in  both  IX.  and  X. . . 


ex  103 


-  28 

-  27 

-  27 


Two  other  holes  similarly  treated  gave: 


exlC 

-  14 

-  17 

Apparently,  therefore,  large  discrepancies  are  not  produced  in  this  way. 


358  GEOLOGY  OF  THE  COMSTOCK  LODE. 

Of  course  all  these  experiments  are  only  intended  to  furnish  estimates 
as  to  the  probable  magnitude  of  disturbances  of  an  analogous  kind,  which 
may  possibly  have  influenced  the  data  given  above. 

Mr.  E.  Kittler^  has  recently  commenced  a  new  study  of  the  question  of 
potential  difference  due  to  the  contact  of  liquids.  From  a  large  number  of 
careful  experiments  he  finds  electromotive  forces  between  them  far  exceed- 
ing, as  a  rule,  those  met  with  in  the  measurements  of  earth  currents  here 
described.     These  forces,  however,  obey  the  law  of  Volta's  potential  series. 

From  all  these  considerations,  it  seems  to  follow  that  in  the  present 
investigation  the  discrepancies  due  to  the  presence  of  different  liquids  in 
the  rocks  have  been  eliminated  to  a  great  extent.  Certainly  their  effect 
can  hardly  be  estimated  as  much  greater  than  a  few  thousandths  of  a  volt. 
It  is  obvious,  moreover,  that  the  use  of  simple  metallic  contacts  (plates  and 
gads)  is  under  all  conditions  unsafe.  To  this  is  to  be  added  the  fact  that 
metalhc  plates  are  never  identical  in  their  electrical  properties,  and  that  their 
difference  (as  large  effects  of  polarization  are  also  included  therein)  cannot 
be  eliminated  by  such  a  process  of  commutation  as  was  employed. 

The  phenomenon  of  conduction  of  rocks  being  essentially  hydro-electric, 
the  determination  of  the  thermo-electric  power  earth|copper,  for  which  it 
was  at  first  thought  the  high  temperature  on  the  lower  levels  of  the  Com- 
STOCK  Lode,  in  comparison  with  those  at  the  sui-face,  would  offer  excellent 
facilities,  has  no  further  interest.  No  attempt  of  this  kind  was  therefore 
made. 

If  the  hole  drilled  for  the  reception  of  the  temiinals  be  regarded  as 
a  cylinder  with  a  hemispherical  base,  the  directrix  of  the  former  as  tangent 
to  the  spliere  corresponding  to  the  latter,  its  axis  as  normal  to  the  plane 
face  of  the  drift,  approximate  values  may  be  derived  for  the  specific  resist- 
ance of  the  rock  met  with.  Let  h  be  the  height  of  the  cylinder,  a  the  common 
radius  of  both  the  latter  and  the  hemisphere.  Let  r  be  the  radius  of  any 
similar  figure,  the  axis  of  whose  cylinder  and  centerof  hemisphere  coincide 
with  those  of  the  hole.  Finally,  let  o  be  the  specific  resistance  of  the  rock, 
or  the  resistance  in  ohms  between  opposite  faces  of  a  cubic  centimeter. 

'  E.  Klttler :  "  Ueber  Spanniingsdifferenzeii,  etc."    Wied.  Ann.,  XTI..  p.  .'i72  et  seq.,  1881. 


ELECTRICAL  ACTIVITY  OF  OEE  BODIES.  359 

The  elementary  resistance  of  a  shell  at  the  distance  r  from  the  axis  and 

of  the  thickness  (Zr,  is  then 

T         a         dr 


'2  7r  r{r-\-hy 

and,  therefore,  the  resistance  of  the  layer  of  rock  between   coaxial  and 
concentric  figures,  the  inner  radius  being  a,  the  outer  r,  is 


['"]:  -^ 


_  iia+h)r 


the  symbol  \^v^   being  used  to  express  the  resistance  of  the  layer  of  rock 

between  the  similar  surfaces  just  defined.  If  r  is  allowed  to  increase  to 
infinity,  approximate  values  for  a  can  be  determined  from  the  data  given 
above  for  the  resistances  of  the  circuits,  and  the  known  dimensions  of  the 
holes.    In  this  way  it  appears  that  the  mean  value  of  this  quantity  was  about 

0-  =  40,000, 
whereas  values  as  high  as  500,000  and  as  low  as  20,000  were  met  with. 
From  the  invariable  presence  of  moisture,  however,  these  figures  possess 
only  minor  interest. 

If  the  resistance  of  layers  of  rock  between  consecutive  similar  sur- 
faces be  compared,  the  same  notation  being  again  employed,  in  round  num- 
bers, 

w~\  \w']  [w] 

Jl?   _  A  C  .  L      JlOO    _  A  A7  .  k_  Jj 


=  0,6;       ^^  =  0.07;      "-^S  =0.007,  etc., 


]io  — "■'-'>  r     -lliiO   ".vi,        r-     -lie 


all  dimensions  being  expressed  in  centimeters,  and  a  being  1.2  cm.;  whence 
it  follows  that  the  resistance  of  coaxial  and  concentric  layers  decreases, 
though  hardly  as  rapidly  as  might  be  desirable.  In  point  of  fact,  however, 
the  convergence  is  more  rapid  than  this  approximate  calcvilation  indicates. 
A  drift  may  with  greater  accuracy  be  regarded  as  a  cylindrical  tunnel.  Into 
the  sides  of  which  the  contact  holes  have  been  drilled,  with  their  axes  at 
right  angles  to  that  of  the  drift.  Now,  it  is  obvious  that  as  r  (in  the  former 
signification)  increases,  the  values  of  dw  will  in  this  case  decrease  more 
rapidly  than  in  the  previous  one;  this  because  the  superficial  area  of  the 
infinitesimally  thin  shell  increases  much  more  rapidly.     The  actual  analysis, 


360  GEOLOGY  OF  THE  COMSTOOK  LODE. 

however,  is  unnecesrary  here.  The  points  of  greatest  interest  have  already 
been  sufficiently  illustrated  by  what  precedes. 

Earth-currents. — A  second  important  consideration  relative  to  the  causes 
which  might  have  produced  discrepancies  in  the  present  investigation  is  the 
effect  to  be  ascribed  to  earth-currents.  Although  numbers  of  experiments 
have  been  made  in  different  parts  of  the  world  as  to  the  magnitude  and 
direction  of  such  currents,  I  am  unable  to  estimate  their  effect  in  this  case, 
especially  as  the  constants  for  the  currents  probably  vary  with  the  position 
of  the  field  of  observation  on  the  surface  of  the  earth.  Most  observers 
have  availed  themselves  of  telegraphic  connections  between  points  very 
many  miles  apart.  Matteuci,^  I  believe,  was  the  only  one  who  laid  a  care- 
fully insulated  line  especially  for  this  purpose,  and  it  is  to  his  investigation 
that  we  can  with  greatest  advantage  refer.  Yet,  though  his  points  were  at  a 
distance  of  six  kilometers  apart,  the  currents  obtained,  so  far  as  can  be  seen, 
were  certainly  not  much  larger  than  those  here  recorded  If,  however,  the 
variation  of  potential  in  the  above  experiments  were  due  to  some  normal, 
non-local  cause,  it  would  be  fair  to  assume  a  linear  change  of  potential 
with  distance  throughout  the  comparatively  small  area  in  which  the  experi- 
ments were  made.  Such  is  by  no  means  the  case.  In  fact,  some  of  the  largest 
variations  observed  occur  within  distances  of  a  few  hundred  feet,  while 
elsewhere  a  range  of  1,000  feet  is  without  marked  alteration  of  potential. 
It  is  probable,  therefore,  that  earth-currents  have  not  perceptibly  affected 
the  results.^ 

Drill-holes. — The  angular  and  somewhat  irregular  curves  (Figs.  28,  30, 
32)  might  give  rise  to  a  suspicion  that  the  difference  of  potential  observed 
is  in  some  way  to  be  ascribed  to  the  accidental  condition  of  the  holes  them- 
selves. A  priori,  therefore,  the  presence  of  little  pieces  of  steel,  worn  or 
broken  off  from  the  drill,  crystals  of  iron  pyrites,  particles  of  ore,  etc.,  in 
the  walls  of  the  hole  should  not  be  disregarded.  That  such  material  is, 
however,  entirely  without  disturbing  effect  will  be  seen  from  the  following 
experiments : 

'  Ch.  Matteuci :  "  Sur  les  courants  ^leotriques  de  la  terre."  Ann.  d.  Chim.  et  de  PhyB.,  [4],  T.  IV., 
p.  177,  18ti5;  ihid.  [4],  T.  X.,  p.  148,  1867. 

^Temporary  disturbances,  such,  for  instance,  as  are  due  to  atmospheric  induction,  are  obviously 
without  influence  in  the  present  case.  Inductive  action,  moreover,  is  hardly  to  be  expected  from  the 
clear,  dry  air  of  Nevada. 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


361 


In  a  particular  case  the  intensity  «i  obtained  by  connecting  two  holes 

in  the  ordinary  manner  was 

ii  =  101:10«. 

A  thin  strip  of  platinum  was  subsequently  introduced  into  one  of  the  holes 
and  firmly  pressed  against  its  sides.     The  intensity  ij  then  measured  proved 

to  be 

io  =  99:10«; 

or,  ^practically,  the  same  as  before.     An  eifect  due  to  the  platinum  was  there- 
fore absent. 

Two  holes,  about  18  inches  apart,  were  drilled  in  solid  rock  and  con- 
nected as"  usual.  The  measurements  made  for  difference  of  potential,  by 
the  original  method,  gave,  in  four  successive  experiments,  different  bags 
being  used  for  each, 

1)  e  =  -f  1:10^ 

2)   eir:— 1:10' 

3)   e  =  ±0:10^ 

4)  e  =  ±0:10^ 

or  zero,  as  from  the  proximity  of  the  holes  it  ought  to  be. 

Finally,  the  potential  of  a  number  of  points  lying  between  Nos.  V.  and 
VII.,  on  the  600-foot  level  of  the  Richmond  mine,  was  determined.  A  zero 
method  being  used,  it  was  only  necessary  to  put  the  terminal  bags  in  con- 
tact with  the  rock  at  the  points  chosen,  by  allowing  them  to  recline  against 
the  wall.  Care  was  taken  to  prevent  any  part  of  the  copper  wire  from 
touching  it.  Two  points,  A  and-B,  were  thus  established  between  VII.  and 
VI. ;  three,  C,  D,  and  E,  between  VI.  and  V.  The  following  table  gives  the 
results,  the  potential  of  No.  VIII.  being  put  equal  to  zero,  as  before: 

Table  XX. 


No. 

Pointa. 

ex  103 

Distance 
from  VII. 

Kem.arka. 

1 

VII 

-    4 

0 

DrUl-hole. 

2 

A 

-  15 

30 

' 

3 

B 

-  44 

60 

4 

VI 

-  30 

87 

Drill-hole. 

5 

C 

-  23 

105 

6 

D 

-     9 

120 

7 

E 

+    2 

140 

8 

V 

+  15 

157 

DriU-hole. 

362 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


+    50; 10' 


0:10' 


50: JO' 


In  Fig.  33  these  results  are  graphically  represented.  It  appears,  not- 
withstanding the  different  kinds  of  contact  at  V.,  VI.,  VII.,  and  at  ^,  i?,  C,  D, 

E,  that  the  progress  of  the 
curve  in  passing  from 
VI.  to  V.  is  continuous. 
The  experiments,  there- 
fore, failed  to  detect  any 
specific  action  due  to  the 
holes.    Nos.  v.,  VI.,  and 

Fig.  33. — Potential  of  intermediate  points.  VII.     Were      especially 

chosen,  because,  as  will  be  seen  from  a  comparison  of  Figs.  30  and  32, 
this  part  of  the  curve  presents  a  curious  and  pronounced  anomaly,  the 
newer  results  differing  verj^  remarkably  from  the  earlier.  It  was  natural 
to  suppose  that,  in  the  time  which  had  elapsed  between  the  two  series  of 
measurements,  hole  No.  VI.  had  in  some  way  been  interfered  with.  The 
results  just  cited,  however,  preclude  such  a  supposition.  Even  larger  masses 
of  metal  seem  to  be  without  marked  effect.  Between  the  date  of  the  earlier 
and  that  of  the  newer  observations,  for  instance,  a  track  had  been  laid 
from  the  vicinity  of  the  hole  No.  I.  to  No.  XV. 

Terminal  bags. — Tlicre  occur  a  few  cases  in  my  notes  in  which,  though  in 
every  other  respect  the  behavior  was  normal,  different  results  were  obtained 
for  the  same  hole  at  nearly  the  same  time,  by  employing  different  bags,  viz.: 
IV.  e  =  0:10*  VII.  e  =  2:10^ 

6  =  2:10'  e=:6:10l 

e  =  6:10' 

These  cases  are  rare,  however,  and  their  effect  is  of  minor  importance. 
More  worthy  of  consideration  are  the  successive  diflPerences  of  potential  due 
to  the  bags  alone  when  employed  for  a  long  period  of  time.  The  quantity 
referred  to  has  already  been  considered  on  page  357,  under  the  symbol  e. 
It  may  readily  be  derived  from  the  tables  for  intensity.  The  following  table 
(XXI.)  probably  contains  good  examples  of  its  consecutive  states,  the  data 
given  being  deduced  from  those  for  the  holes  I.-XIV.  on  the  600-foot  level. 
If  the  bags  are  called  A,  B,  0,  D,  the  electromotive  force  e  between  A  and 
B  may  be  conveniently  represented  hj  A  \  B,  between  A  and  C  hy  A  \  C 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 


363 


etc.  The  values  of  e,  as  derived  both  from  the  direct  and  return  series,  are 
given  in  the  table,  the  latter  being  primed.  Heavy  black  lines  across  the 
table  indicate  either  that  the  bags  were  refilled  or  that  the  experiments  had 
to  be  temporarily  discontinued. 

Table  XXL,  containing  ex  10". 


So. 

Points. 

A|B 

A|C 

AID 

A' IB' 

A'lC 

A' ID' 

1 

I 

-  17 

-  16 

-  15 

-  1 

±  0 

±  0 

2 

II 

-  12 

-  11 

-     8 

_  2 

±  0 

+  1 

3 

i 

m 

IV 

-  16 

-  15 

-  U 

-  1 

±  0 

+  1 
+  1 

+  1 
+  2 

±     0 

+     1 

+     1 

5 

V 

-    1 

-     1 

-     3 

±   0 

+  1 

+  1 

6 

VI 

-     2 

-    2 

_     2 

+  1 

+  2 

-  3 

7 
8 

VII 
IX 

±     0 

—    2 

-     1 

+  0 

+  2 

±  0 

—     2 

-     2 

-     3 

-  1 

+  1 

+  3 

9 

X 

—     1 

-     1 

-     4 

±   0 

+  0 

-  1 

10 

XI 

±     0 

—    1 

-     3 

-  1 

±  0 

±  0 

11 

XII 

—    4 

-     3 

-     5 

-  1 

±   0 

J-  0 

12 

XIII 

-     4 

-    2 

-     4 

+  2 

+   0 

+ 1 

13 

XIV 

-     1 

-     1 

-     2 

-  1 

+ 1 

-  2 

The  successive  values  of  £  are  not  constant,  though  in  the  majority  of 
cases  they  are  so  small  as  to  be  immaterial.  At  times,  however,  values  suffi- 
ciently large  to  be  important  are  reached.  An  exchange  of  terminals  is 
therefore  indispensable,  especially  as  experiments  will  usually  be  sufficiently 
extensive  to  involve  interruptions.  The  gradual  variation  observed  in  the 
value  of  e  can  most  probably  be  referred  to  a  corresponding  change  in  the 
concentration,  etc.,  of  the  solution  (zinc  sulphate)  contained  in  the  bags.  It 
is  hardly  probable  that  it  is  due  to  polarization,  or  a  change  in  the  surface  of 
the  amalgamated  zinc  strips.  It  is  interesting  that,  in  spite  of  the  fact  that 
for  the  holes  I.,  II.,  and  III.  the  electromotive  force  between  the  bags  in  the 
direct  and  return  series  differs  largely,  the  lode-currents  deduced  from  the 
two  sets  of  data  are  practically  equal.     (See  Table  XV.) 

Wire. — In  the  above  experiments  especial  care  was  taken  to  prevent 
errors  due  to  leaks  in  the  wire.  The  galvanometer  was  sufficiently  delicate 
to  register  a  fault  of  this  kind  of  5,000,000  ohms'  resistance  with  certainty. 
As  has  been  mentioned,  every  leak  introduces  an  electromotive  force 
zinc  I  copper;^  hence  the  great  necessity,  notwithstanding  the  fact  that  the 
latter  must  act  through  a  very  great  resistance,  of  avoiding  leakage. 

'For  we  have  the  closed  couple:  Copper  (of  wire);  liquid  (moist  earth,  etc.);  zinc  (of  bag). 


364  GEOLOGY  OF  THE  COMSTOCK  LODE. 

General  remarks. — TliG  opinioii  Has  been  expressed  (page  35 1 )  that  the  field  of 
electric  excitation  is  confined  to  particular  parts  of  the  ore  body.  That  this 
should  be  the  case  is  not  surprising,  as  the  conclusion  has  already  been 
reached  that  contact  between  different  kinds  of  material  is  necessary  for  the 
production  of  currents.  In  the  connection  made  between  chambers  No.  14 
and  No.  15,  as  well  as  in  the  survey  on  the  400  and  500-foot  levels,  the  oi*e 
actually  met  with  was  principally  lead  carbonate,  at  times  stained  with  sul- 
phide and  ferric  oxide.  Now,  disregarding  the  sulphide,  which  is  here  very 
unfavorably  associated,  more  pronounced  electrical  properties  can  hardly  be 
ascribed  to  the  remaining  constituents  of  the  deposit  than  to  the  siirrounding 
rock  itself  For,  judging  from  physical  properties,  cerusite  may  be  regarded 
as  an  insulator  with  as  much  right  as  calcite,  earthy  lead  carbonate  as  lime- 
stone. In  fact,  it  seems  to  follow  that  the  feeble,  though  none  the  less 
positive,  reaction  observed  on  the  600-foot  level  is  already  partially  obscured 
when  the  line  of  points  on  the  500-foot  level  is  reached,  and  would  perhaps, 
coet.  par.,  be  equally  obscured  on  the  700-foot  level.  I  am  also  inclined  to 
infer  that  the  currents  observed  on  the  surface  are  not  due,  or,  rather,  not 
immediately  due,  to  the  deeper  ore  bodies  (Nos.  11,  12,  13,  14,  15,  etc.),  but 
to  the  deposits,  also  of  considerable  size,  occurring  in  what  are  known  as 
the  Lizette  Tunnel  workings.  The  entrance  to  the  latter  is  on  a  level  with 
the  mouth  of  the  shaft,  and  the  ore  masses  are  distributed  in  a  vertical  range 
from  Point  I.  to  a  level  even  above  Point  X.  on  the  surface.  These  ore 
bodies,  throughout  their  extent,  are  comparatively  near  the  line  of  holes 
used  in  the  surface  survey.  It  is,  moreover,  quite  probable  that  an  inti- 
mate connection  exists  between  these  and  the  large  group  of  ore  bodies 
below. 

In  consideration  of  the  statements  made  in  the  foregoing  paragraph, 
and  allowing  as  accurately  as  possible  for  discrepancies,  the  results  thus  far 
reached  may  be  regarded  as  agreeing  well  with  the  fundamental  hypothesis. 


ELECTRICAL  ACTIVITY  OF  ORE  BODIES.  365 


CONCLUDING  REMARKS. 

On  reviewing  the  results  described  it  is  strikingly  evident  that  the 
electromotive  forces  met  with  are  invariably  small,  very  frequently,  indeed, 
quite  at  the  limit  of  the  accurately  measurable.  It  is  true  that  the  elec- 
trically active  material  was  probably  galena,  which,  as  Fox  long  ago  ob- 
served, is  unfavorable  for  observations  like  the  present.  It  is  a  question, 
however,  whether  results  much  larger  than  these  will  generally  be  obtained. 
I  cannot  believe  that  Reich's  earnest  appeal  for  genei'al  research  in  the 
direction  of  electric  prospecting  has  been  altogether  disregarded.  There 
is  much  more  to  lead  one  to  infer  that  many  undertook  the  study  of  the 
question,  but,  disappointed  with  feeble  reactions  and  discordant  results, 
abandoned  the  matter  altogether.  Reich,  at  the  end  of  his  last  paper, 
gives  a  list  of  the  apparatus  desirable,  which,  however,  except  where  the 
action  is  so  intense  as  it  was  found  to  be  in  Cornwall,  and  to  a  less  extent 
at  Freiberg,  would  certainly  be  insufficient. 

The  very  large  currents  obtained  in  the  localities  just  mentioned  ren- 
dered it'not  improbable,  at  the  outstart  of  the  present  investigation,  that 
important  conclusions  might  be  drawn  from  the  results  of  a  minute  mag- 
netic survey^  of  the  interior  of  the  mines,  or  across  the  vein  on  the  surface ; 
and  preparations  for  such  a  purpose  were,  in  fact,  made.  But  the  currents 
obtained  galvanometrically  dictated  the  abandonment  of  this  project. 

The  study  of  the  electric  activity  of  ore  bodies  should  be  carried  out 
on  a  broader  basis  than  was  possible  in  the  present  case,  to  reach  the  best 
results.  A  single  line  of  survey,  or  the  investigation  of  the  variation  of 
potential  in  a  single  drift,  is  far  from  sufficient.  The  endeavor  should  be 
made  to  map  the  equipotentials  as  surfaces  traversing  the  whole  mine,  care- 
fully considering  their  position  and  contour  relatively  to  any  ore  already 
in  sight,  and  their  change  of  form  on  leaving  it.  The  inferences  to  be  drawn 
herefrom  would  certainly  compare  in  value  with  those  of  a  purely  geological 

'  Fox  himself  entertained  an  idea  of  this  kind. 


366  GEOLOGY  OP  THE  COMSTOCK  LODE. 

character,  even  though  dependence  must  be  placed  on  the  latter  for  a  com- 
plete interpretation  of  the  results. 

Furthermore,  it  will  be  desirable  to  carry  out  Fox's  original  idea,  namely, 
of  investigating  the  electrical  properties  of  ores  and  those  minerals  of  the 
heavy  metals  which  are  usually  found  associated  with  them;  not  that  the 
results  of  such  an  investigation  could  ever  furnish  a  clew  as  to  the  par- 
ticular ore  to  which  an  observed  electric  effect  is  due  (it  is  here  that  our 
knowledge  of  the  locality  must  aid  us),  but  that  the  class  of  ores,  in  pros- 
pecting for  which  an  electric  method  would  be  peculiarly  applicable,  could 
thus  be  defined.  The  knowledge  we  possess  of  the  conductivity  and  the ' 
position  of  ores  in  the  electrical  scale  is  largely  the  result  of  experiments 
made  a  long  time  ago.  Recent  observers  have  made  but  few  quantitative 
additions,  and  even  these — probably  from  improperly  chosen  methods — are 
frequently  discordant. 

The  method  which  has  been  described  seems  to  me  especially  worthy 
of  consideration,  from  the  fact  that  by  means  of  it  an  electric  survey,  made 
on  the  surface,  may  detect  not  only  the  presence  but  also  the  approximate 
position  of  ore  bodies  under  ground.  With  such  an  end  in  view  the  exper- 
iments should  be  extended  over  a  large  area,  and  the  potential  at.  all  por- 
tiqns  of  the  surface  determined.  Suppose,  now,  that  at  each  point  of  the 
projection  of  the  latter  on  a  fixed  horizontal  plane,  a  vertical  line  is  erected, 
of  a  length  proportional  to  the  earth-potential  at  this  point.  The  ends  of 
all  such  lines  together  make  up  a  second  imaginary  surface,  coextensive 
with  the  first,  which  will  represent  the  electric  state  graphically  at  each 
point  of  the  territory  over  which  the  survey  has  been  carried. 

The  effect  of  normal  earth-currents  would  then  express  itself  in  the 
progress  and  contour  of  the  imaginary  surface  as  a  whole,  and  would  not 
destroy  its  regularity.  If  its  extent  is  not  too  large,  this  (normal)  surface 
will  be  a  more  or  less  inclined  plane.  As  it  has  been  observed  that 
earth-currents  are  not  constant,  even  for  short  periods  of  time,  the  latter  is, 
moreover,  to  be  regarded  as  slowly  oscillating,  more  or  less  parallel  to  itself, 
about  a  certain  temporarily  fixed  position  of  equilibrium.  But  it  is  prob- 
able that  the  limiting  positions  of  the  plane  are  so  near  to  one  another  that 
for  this  purpose  they  may  be  regarded  as  coincident. 


BLECTEICAL  ACTIVITY  OF  OEE  BODIES.  367 

Local  action,  however,,  in  contrast  to  the  foregoing,  would  probably 
manifest  itself  locally  in  the  imaginary  surface,  as  a  hillock  or  depres- 
sion. It  is  to  such  anomalies  that  attention  should  subsequently  be 
directed,  the  electric  activity  of  ore  bodies,  differences  of  potential  of  liquids 
in  contact,  and  Matteuci's  effects^  constituting  the  salient  points  to  be  consid- 
ered. In  the  interest  of  expeditious  work  all  measurements  should  be  made 
electrometrically,  the  lines  of  survey  radiating  from  a  central  point  (P.  C.) 
the  potential  of  which  is  arbitrarily  taken  as  zero.^ 

'  Those  mentioned,  p.  352. 

^The  prosecution  of  the  experiments  described  in  this  chapter  was  aided  by  the  cordial  coopera- 
tion of  Messrs.  Patton,  Lamb,  anil  Ballard,  of  Virginia  City,  and  Messrs.  Eickard,  Westcott,  Harris, 
and  Bryan,  of  Eureka,  Nevada.  The  work  is  also  indebted  to  Professor  Micliie,  of  West  Point,  for  the 
loan  of  a  Rowland  magnetometer  made  by  Mr.  Wm.  Grunow,  of  New  York. 


CHAPTER   XI. 

SUMMARY. 

BY  GEORGE  F.  BECKER. 

Purpose  of  this  chapter. — A  verj  lai'gc  portion  of  the  foregoing'  pages  is  neces- 
sarily occupied  by  detailed  descriptions,  written  to  enable  readers  to  judge 
whether  the  facts  warrant  the  opinions  expressed,  and  by  discussions  of 
a  somewhat  technical  character.  There  may  be  those,  however,  who 
will  be  interested  to  know  in  brief  what  conclusions  have  been  reached, 
but  who  have  no  inclination  to  undertake  the  somewhat  serious  task  of 
weighing  the  evidence  adduced,  and  of  following  the  arguments  in  detail; 
and  for  such  tlie  present  chapter  is  written,  but  with  the  proviso  that  full 
and  fully  qualified  statements  are  to  be  found  in  the  body  of  the  report,  and 
there  only. 

History  and  statistics. — No  more  condcused  statement  of  the  technical  and 
economical  relations  of  the  Comstock  mines  can  be  presented  than  that 
which  is  given  in  Chapter  I.,  itself  a  meager  abstract  of  reports  which  will 
appear  hereafter;  nor  is  it  necessary  further  to  reduce  the  digests  of  the 
previous  memoirs  on  the  Lode  which  constitute  Chapter  II.  In  some  re- 
spects the  present  volume  is  a  tribute  to  the  acumen  of  preceding  observers, 
upon  whose  investigations  that  here  described  is  to  a  great  extent  built  up. 
That  the  recent  development  of  the  science  of  microscopical  petrography 
and  the  immensely  increased  facilities  for  observation,  due  to  the  extension 
of  the  mine  workings,  should  have  led  to  some  views  different  from  those 
heretofore  entertained  concerning  the  geology  of  the  District  is  anything 
but  surprising. 

368 


SUMMARY.  369 


LITHOLOGY. 


Importance  of  the  rock  determinations. — In  areas  SO  largely  covercd  bj  massive 
rocks  as  the  Washoe  District,  lithological  determinations  form  the  neces- 
sary preliminary  to  geological  investigation,  for  few  points  in  the  history  or 
the  structure  of  such  a  region  are  independent  of  the  character  of  the  rocks 
involved.  Moreover,  the  economical  importance  of  the  District,  the  ob- 
scure character  of  some  points  in  its  geology,  and  the  great  weight  of  the 
authorities  whose  investigations  had  already  been  published,  made  it  essen- 
tial that  the  work  done  under  the  new  United  States  Geological  Survey 
should  be  supported  by  the  strongest  and  most  detailed  evidence.  The 
collections  embrace  over  2,600  specimens  and  500  microscope  slides.  The 
locality  of  each  specimen  was  fixed  with  great  care  on  the  maps  at  the 
time  of  collection,  and  no  time  or  pains  was  spared  in  preparing  the  geo- 
logical maps  and  sections.  In  laying  down  the  various  formations  the 
microscope  was  in  constant  use,  slides  being  ground  as  the  occasion  arose, 
and  the  results  obtained  from  them  finding  immediate  application  in  the 
extension  of  the  work. 

The  area  in  which  the  Comstook  lies  is  characterized  by  a  wide-spread 
and  profound  decomposition  of  the  rock  masses,  and  a  study  of  the  lithology 
of  the  District  resolves  itself  primarily  into  an  investigation  into  decom- 
position. In  spite  of  the  most  painstaking  choice  of  specimens,  there  is  not 
one  in  fifty  of  those  collected  underground  which  contains  a  particle  of 
either  of  the  characteristic  bisilicates  or  of  the  lithologically  equivalent 
unisilicate,  mica,  secondary  minerals  replacing  them  throughout.  Even  the 
feldspars  are  rarely  intact,  and  are  sometimes  wholly  decomposed.  When 
the  steps  of  these  processes  of  degeneration  are  once  understood,  however, 
it  is  comparatively  easy  to  infer  the  original  composition  and  structure  of 
the  rock.     Some  of  the  results  obtained  are  the  following: 

Decomposition. — Homblende,  augite,  and  mica  generally  pass  into  a  chlo- 
ritic  mineral,  which,  so  far  as  can  be  judged  by  any  optical  tests  now  known, 
is  almost  without  exception  the  same,  from  whichever  of  the  ferro-magne- 
sian  silicates  it  may  have  originated.  This  chlorite  is  generally  green,  but 
in  especially  compact  masses  appears  greenish-brown  under  the  microscope. 
24  c  L 


370  GEOLOGY  OF  THE  COMSTOCK  LODE. 

It  is  strongly  dichroitic,  but  except  in  dense  masses  appears  nearly  black 
between  crossed  Nicols.  It  is  fibrous,  often  spherolitic,  and  invariably  ex- 
tinguishes light  parallel  to  the  direction  of  the  fibers.  It  thus  bears  a  con- 
siderable resemblance  to  fibrous  green  hornblende,  but  the  cases  are  very 
rare,  if  they  actually  occur,  in  which  a  careful  examination  will  not  serve 
to  discriminate  between  the  minerals.  This  chlorite  is  decidedly  soluble. 
It  occurs  in  veinlets  and  difi'used  through  the  groundmass  and  through  other 
minerals  when  these  have  become  pervious  through  decomposition.  It  is 
especially  striking  as  an  infiltration  in  altered  feldspars,  where,  of  course,  it 
is  readily  visible.  All  the  stages  can  be  traced,  from  the  first  inconsiderable 
attack  upon  the  bisilicates  or  the  mica  through  instances  in  which  chlorite 
occurs  wholly  or  almost  wholly  as  admirable  pseudomorphs  after  the  ferro- 
magnesian  silicates,  and  up  to  cases  in  which  the  secondary  mineral  is  wholly 
diffused  through  the  mass  of  other  products  of  decomposition. 

Epidote  is  usually  in  Washoe  a  product  of  the  decomposition  of  chlo- 
rite. Comparatively  very  few  occurrences  of  epidote  are  explicable  on  the 
supposition  that  the  mineral  is  the  direct  result  of  the  decomposition  of  the' 
primary  siHcates;  none  are  inexplicable  on  the  supposition  that  chlorite 
represents  an  intermediate  stage  in  the  alteration,  and  hundreds  of  cases 
show  beyond  question  that  epidote  develops  in  chloritic  masses,  sending 
characteristic  denticles  and  fagot-like  ofi"shoots  into  the  comparatively  homo- 
geneous chlorite.  Several  drawings  illustrating  these  processes  are  shown 
in  Plate  II.  They  are  photographic  in  their  fidelity.  Epidote,  too,  is  pos- 
sibly soluble  to  a  very  slight  extent,  but  certainly  far  less  so  than  chlorite. 
The  veinlets  of  epidote  are  often,  though  perhaps  not  always,  a  result  of 
the  alteration  of  chlorite.  No  evidence  has  been  obtained  that  feldspars 
are  ever  converted  into  epidote,  and  the  dissemination  of  fresh  hornblende 
particles  in  feldspars  in  any  considerable  number  has  not  been  observed. 
In  many  cases,  on  the  other  hand,  it  can  be  shown  that  feldspars  have  been 
impregnated  with  chlorite,  from  which  epidote  has  afterwards  developed. 
Chlorite  does  not  always  change  to  epidote,  and  appears  often  to  be  replaced 
by  quartz  and  calcite.  This  is  frequently  visible  in  shdes  which  also  show 
its  alteration  to  epidote.  No  certain  evidence  of  the  alteration  of  epidote 
has  been  met  with. 


STJMMAET.  371 

In  the  decomposition  of  the  feldspars,  the  first  stage  appears  to  be  the 
formation  of  calcite.  This  sometimes  leaches  out,  leaving  small  irregular 
cavities,  and  these  cavities  are  not  infrequently  filled  with  liquid,  some- 
times carrying  a  bubble,  which  is  commonly  stationary,  but  occasionally 
active.  Thus  secondary  liquid  inclusions  are  formed,  which  may  mislead 
in  the  diagnosis  of  a  rock.  Primary  liquid  inclusions  are  either  more  or 
less  perfect  negative  crystals  or  vesicular  bodies.  The  vesicles  often  assume 
strange  forms  through  pressure,  such  as  are  often  observed  in  air-bubbles 
in  the  balsam  of  a  slide,  but  their  outlines  are  composed  of  smooth  curves. 
The  secondary  fluid  inclusions  are  bounded  by  jagged  lines.  Inclusions  of 
this  kind  are  never  met  with  unaccompanied  by  other  evidences  of  decom- 
position, and  thus  are  abundant  in  the  altered  outer  crust  of  andesite  masses, 
the  inner  portions  of  which  show  none  of  them.  There  is  every  reason  to 
suppose  that  such  secondary  inclusions  would  form  in  older  rocks,  and  it  is 
believed  that  many  of  them  have  been  detected  in  the  pre-Tertiary  erup- 
tives  of  the  District;  but  in  the  older  rocks  their  secondary  character  can 
only  be  suspected,  not  proved. 

Kaolin  possesses  so  few  characteristic  optical  properties  that  it  is  not 
recognized  with  ease  or  certainty  under  the  microscope.  No  kaolin  has  been 
identified  in  the  Washoe  rocks,  and  while  it  is  by  no  means  asserted  that 
they  contain  none,  it  seems  hardly  possible  that,  had  it  formed  a  prominent 
constituent,  it  would  have  escaped  observation.  The  presence  of  enormous 
masses  of  "clay"  on  the  Comstock  does  not  prove  the  existence  of  much 
kaolin,  for  the  so-called  clays  of  veins  are  largely  attrition  mixtures. 

An  increase  in  volume  appears  to  accompany  the  decomposition  of  the 
"Washoe  rocks.  This  is  perceptible  where  dense  masses,  such  as  the  more 
compact  andesites,  are  subjected  to  the  process.  Angular  blocks  are  then 
converted  into  a  series  of  concentric  shells  of  comparatively  soft  matter, 
which  approach  the  spheroidal  shape  more  and  more  as  the  diameter  dimin- 
ishes.^ Often  a  nodule  of  undecomposed  rock  is  found  at  the  center,  and 
such  masses  aff"ord  the  very  best  opportunity  for  studying  the  macroscopical 
appearances  resulting  from  degeneration.     When  the  attacked  mass  is  large, 

'Prof.  R.  Pumpelly  has  described  the  course  of  decomposition  almost  in  the  same  words,  in  his 
paper  "  On  the  Relation  of  Secular  Rock-disintegration  to  Loess,  etc."  (Amer.  Journ.  XVII.,  1879, 136.)  I 
did  not  happen  to  see  Professor  Pumpelly's  paper  until  after  this  passage  was  written. 


372        GEOLoaY  OP  the  comstock  lode. 

erosion  often  exposes  the  fresh  core,  wliich  then,  oflfering  greater  resistance, 
projects  as  a  "  cropping,"  or,  if  it  has  an  elongated  form,  it  protrudes  hke  a  dike 
above  the  surrounding  country.  As  the  tendency  of  the  mere  action  of  atmos- 
pheric agencies  is  to  the  production  of  ferric  hj^drate  rather  than  of  chlorite 
from  the  bisilicates,  the  first  impression  which  such  a  mass  produces  is  that 
of  an  older  and  a  younger  rock  in  conjunction.  Nevertheless,  sufficiently 
thorough  examination  will  reveal  a  transition.  When  the  rock  is  not  solid, 
but  bx'ecciated  or  loose-grained,  sufficient  space  often  seems  available  to 
permit  the  requisite  increase  of  volume  without  disintegration.  Large  and 
often  i^rominent  masses  of  very  strongly  cohesive  decomposition-products 
derived  from  breccia  are  common  in  the  District. 

The  mineralogical  character  and  the  microscopical  phenomena  of  de- 
composition seem  to  be  identical  in  the  different  rocks.  Those  refined  mani- 
festations of  physical  character  by  which  it  is  so  often  possible  to  discriminate 
between  older  and  younger  rocks,  and  between  the  various  rock  species 
when  fresh,  are  nearly  or  quite  obliterated  by  the  decomposition  process, 
which  impresses  its  own  character  on  the  pi'oduct. 

Rocks  of  the  District. — The  rocks  occurring  in  the  Washoe  District  are  gran- 
ite; metamorphic  schists,  slates,  and  limestone;  eruptive  diorite  of  three 
varieties;  metamorphic  diorite ;  quartz-porphyry ;  an  older  and  a  younger 
diabase;  an  older  and  a  younger  hornblende-andesite ;  augite-andesite,  and 
basalt.^  Chapter  III.  contains  a  discussion  of  each  of  these  rocks  and  a 
detailed  description  of  about  seventy-five  slides,  and  is  well  illustrated. 
Here  they  can  be  dismissed  with  a  very  few  remarks. 

'The  signification  attached  to  these  names  has  varied  somewhat  as  the  science  of  lithology  has 
progressed.    Some  of  the  main  points  of  their  definitions  as  here  understood  are  as  follows : 

Granite,  pre-Tertiarynon- vitreous  crystalline  rock,  of  which  the  principal  constituents  are  ortho- 
clase,  quartz,  and  mica  or  hornblende. 

Diorite,  pre-Tertiary  non- vitreous  crystalline  rock,  of  which  the  main  constituents  are  plagioclase 
and  hornblende.     It  may  or  may  not  contain  quartz. 

Quartz-porphyry,  pre-Tertiary  glass-bearing  porphyritic  rock,  of  which  the  main  constituents 
are  orthoclase,  quartz,  and  hornblende  or  mica. 

Diabase,  pre-Tertiary,  more  or  less  porphyritic  rock,  of  which  the  main  constituents  are  plagio- 
clase and  augite. 

Andesite,  Tertiary  or  post-Tertiary,  glass-bearing,  more  or  less  porphyritic  rock,  of  which  the 
main  constituents  are  plagioclase  and  hornblende,  mica,  or  augite.  The  andesites  in  which  augite  is 
the  characteristic  bisilicate  appear  to  be  separate  eruptions,  while  mica  and  hornblende  replace  one 
another  to  a  variable  extent  in  the  same  eruption.    In  the  andesites  feldspar  predominates. 

Bas.alt,  Tertiary  or  post-Tertiary  plagioclase  augite  rock,  with  predominant  augite,  usually  char- 
acterized by  the  presence  of  olivine. 


SUMMARY.  373 

Concerning  the  granite  and  basalt  there  has  scarcely  been  a  question. 
They  are  eminently  characteristic  occurrences.  The  metamorphic  diorite 
sometimes  resembles  eruptive  diorite,  and  has  been  taken  both  for  diorite 
and  granite ;  usually  it  bears  some  resemblance  to  augite-andesite  or  basalt, 
and  has  been  determined  microscopically  as  an  unusual  variety  of  the 
latter  rock.  It  is  composed  essentially  of  oligoclase  and  hornblende.  The 
hornblende  was  originally  coloi'less,  but  through  some  chang-e  (perhaps 
absorption  of  water)  it  is  in  large  part  converted  into  an  intensely  green 
variety.     The  hornblende  polarizes  in  unusually  intense  colors. 

The  quartz-porphyry  underlies  both  hornblende-andesite  and  diabase. 
The  microscope,  Thoulet's  method  of  separation,  and  analysis,  show  that 
the  predominant  feldspar  is  orthoclase.  It  is  characterized  by  the  associa- 
tion of  liquid  and  glass  inclusions  usual  in  quartz-porphyry,  to  which  also 
the  groundmass  corresponds.  In  one  locality,  near  the  Red  Jacket,  the 
quartz  is  nearly  suppressed,  and  the  rock  is  excessively  fine-grained.  It  is 
a  felsitic  modification  of  the  ordinary  variety.  This  rock,  which  Baron  v. 
Richthofen  determined  correctly,  has  since  been  called  quartz-propylite, 
dacite,  and  in  its  felsitic  modification  rhyolite.  Most  of  the  quartz-porphyry 
is  greatly  decomposed. 

The  eruptive  diorite  is  sometimes  granular,  sometimes  porphyritic.  In 
the  porphyritic  diorite  mica  frequently  predominates  over  hornblende. 
Quartz  is  irregularly  disseminated  through  the  rock.  In  the  granular  dio- 
rite the  hornblende  is  sometimes  green  and  fibrous,  sometimes  brown  and 
solid.  In  some  cases  it  can  be  shown  that  the  latter  variety  of  hornblende 
is  altered  to  the  former,  and  possibly  this  is  ordinarily  the  case.  Augite  is 
not  uncommon,  and  a  part  of  the  fibrous  green  hornblende  is  very  likely 
uralite,  but  in  the  granular  rock  the  outlines  of  the  crj^stalline  grains  are 
rarely  sufficiently  regular  to  determine  this  point.  In  the  porphyritic  dio- 
rites  the  fresh  hornblende  is  always  brown.  Even  in  this  latter  variety  of 
thediorites  well-developed  feldspars  are  rare.  The  porphyritic  diorites  have 
for  the  most  part  been  regarded  as  propylite,  and  some  occurrences  of  the 
granular  rock  have  been  classed  in  the  same  way.  Some  of  the  fresher 
porphyritic  diorites  have  been  mistaken  for  andesites,  the  resemblance  to 
which  is  occasionally  strong. 


374  GEOLOGY  OF  THE  COMSTOCK  LODE. 

The  older  diabase  is  porphyritic,  and  almost  the  whole  of  it  is  in  a  very 
advanced  stage  of  decomposition.  When  fresh,  it  considerably  resembles 
an  augite-andesite;  its  groundmass,  however,  is  thoroughly  crystalline  and  it 
contains  no  glass  inclusions,  but  frequent  fluid  ones;  the  augites,  too,  show 
both  pinacoidal  and  prismatic  cleavages,  and  a  tendency  to  uralitic  decom- 
position. It  is  also  manifestly  older  than  the  other  diabase.  An  important 
characteristic  is  the  lath-like  development  of  the  porphyritic  feldspars,  for 
in  cases  of  extreme  decomposition  of  the  bisilicates  this  characteristic  at 
least  serves  to  suggest  whether  the  rock  is  dioritic  or  diabasitic.  The  older 
diabase  has  been  considered  as  propylite  or  andesite,  according  to  the  stage 
of  decomposition.  The  younger  diabase  ("black  dike")  is  very  highly 
crystalline  and  not  porphyritic.  It  is  bluish  when  fresh,  but  in  course  of  a 
few  hours  turns  to  a  smoky  brown.  It  is  identical  with  many  of  the  dia- 
bases of  the  New  England  and  the  Middle  States. 

The  older  hornblende-andesite  and  the  augite-andesite  where  fresh  are 
typical  rocks  macroscopicall}^  and  microscopically.  When  decomposed  they 
have  been  taken  for  propylite.  The  younger  hornblende-andesite  which 
overlies  the  augite-andesite  is  a  cross-grained,  soft,  often  reddish  or  pur- 
plish rock,  with  large  glassy  feldspars.  It  has  always  been  supposed  to  be 
trachyte;  but  when  endeavoring  to  determine  the  different  species  of  feld- 
spar under  the  microscope,  I  was  unable  to  include  an}'  satisfactorily  deter- 
minable sanidins  in  the  list.  Dr.  G.  W.  Hawes  was  kind  enough  to  under- 
take the  separation  of  the  feldspars  by  Thoulet's  method,  and  analyses  of 
the  feldspars  were  made  by  Mr.  F.  P.  Dewey.  The  specimen  selected  was 
the  most  trachytic  in  appearance,  that  of  Mount  Rose,  but  no  feldspar  what- 
ever was  found  corresponding  either  physically  or  chemically  to  orthoclase. 
There  is  much  reason  to  believe  that  trachyte  occurs  less  often  than  had 
been  supposed  in  the  Great  Basin  area. 

Apart  from  the  effects  produced  by  decomposition  the  Washoe  rocks 
are  typical  of  their  kind,  and  correspond  to  representative  specimens  of  the 
same  species  from  other  parts  of  the  world,  even  in  the  minutiae  of  miner- 
alogical  composition  and  physical  structure.  This  persistence  of  rock  types 
in  minor  features,  which  would  seem  to  be  fortuitous,  or  at  least  unessential, 
is  one  of  the  most  remarkable  facts  established  by  microscopical  lithology, 


STJMMAEY.  375 

and  indicates  a  repetition  of  absolutely  identical  physical  and  chemical  con- 
ditions at  distant  points,  which  is  far  from  having  received  an  adequate  ex- 
planation. 

propyiite. — Thc  prcscnt  investigation  of  the  geology  of  the  Washoe  Dis- 
trict has  failed  to  establish  the  existence  of  propyiite.  Full  proof  of  this 
responsible  statement  cannot  of  course  be  given  in  this  summary  of  results. 
It  consists  in  a  process  of  exhaustive  elimination.  A  study  of  each  of  the 
rocks  of  the  District,  in  all  stages  of  decomposition,  has  led  to  the  identi- 
fication of  all  of  them  with  other  and  previously  recognized  species.  The 
reduction  of  rocks  of  originally  different  aspect  to  an  apparently  uniform  char- 
acter by  chl  critic  decomposition  is  strikingly  evinced  by  a  mere  list  of  the 
species  in  the  District,  which  have  been  grouped  under  the  terms  "propy- 
iite" and  "  quartz-propylite."  These  are  granular  diorite,  porphyritic  diorite, 
diabase,  quartz-porphyry,  hornblende-andesite,  and  augite-andesite.  The 
peculiar  habitus  which  is  always  referred  to  in  descriptions  of  propyiite 
appears  to  consist  in  the  impellucidity  of  the  feldspars,  the  green  and  fibrous 
character  of  the  hornblende,  the  greenish  color  which  often  tinges  feldspars 
and  groundmass,  and  a  certain  blending  of  the  mineral  ingredients.  The 
impellucidity  of  the  feldspars  (which  surprisingly  alters  the  appearance  of 
rocks  originally  containing  transparent  unisilicates)  is  due  to  incipient 
decomposition,  especially,  as  it  seems,  to  the  extraction  of  calcite.  The 
"green  hornblendes"  are  simply  pseudomorphs  of  chlorite  after  hornblende 
or  augite,  as  the  case  may  be.  Excepting  the  granular  diorite,  not  one  of 
the  rocks  from  which  propyiite  forms  has  ever  been  found  in  the  Washok 
District  containing  primitive  green  hornblende,  though  uralite  is  common. 
The  other  characteristics  are  due  to  the  diffusion  of  chlorite  and  the  forma- 
tion of  epidote  from  it.  The  description  of  propyiite  as  a  species  arose  from 
the  erroneous  determination  of  chlorite  as  green  hornblende — a  very  natural 
mistake  before  the  microscope  was  brought  to  bear  on  the  subject,  since  even 
with  that  instrument  the  same  error  may  be  committed  if  color  and  dichroism 
are  exclusively  relied  upon  as  diagnostic  tests.  The  microscopical  charac- 
teristics of  propyiite  are  illusory.  Finely  disseminated  hornblende  in  the 
groundmass  of  a  Washoe  rock  is  very  rare,  and  far  rarer  is  the  presence  of 
particles  of  hornblende  in  feldspars.    The  propylites  contain  glass  inclusions 


376  GEOLOGY  OF  THE  COMSTOCK  LODE. 

and  primitive  liquid  inclusions,  or  not,  according-  to  the  rock  from  which 
they  were  derived.  Base  is  rare  in  propylites;  where  it  originally  formed 
a  constituent  of  the  rock,  it  has  for  the  most  part  undergone  devitrification. 

A  reexamination  has  been  made  of  all  the  slides  of  propylites  from 
other  localities  as  well  as  from  the  Washoe  District,  descriptions  of  which 
have  been  published  in  different  government  reports.  These,  too,  can  be 
referred  to  other  rock  species  with  great  probability,  in  spite  of  advanced 
decomposition,  and  I  do  not  hesitate  to  affirm  that  there  is  no  proof  yet  known 
of  the  existence  of  a  pre-andesitic  Tertiary  eruption  in  the  United  States. 

The  term  "propylite"  should  not  be  retained  in  the  nomenclature  of 
American  geology  even  to  express  certain  results  of  decomposition,  for  the 
equally  loose  term  "greenstone"  seems  to  cover  the  same  ground  and  has 
priority. 

A  few  minor  questions  of  interest  were  raised  by  the  microscopic 
examinations,  in  addition  to  those  bearing  directly  upon  the  identification 
of  the  rocks.  Such  are  the  occurrence  of  zonal  plagioclases  and  their  bear- 
ing on  Tschermak's  feldspar  theory;  hornblendes  with  concentric  belts  of 
magnetite,  and  the  indications  they  furnish  as  to  the  conditions  under  which 
"black  borders ''  form,  and  some  other  small  points. 


STEUCTUEAL  RESULTS  OF  FAULTING. 

Evidences  of  faulting. — The  evidence  of  faulting  on  the  Comstock  is  mani- 
fold, and  has  been  recognized  by  all  observers.  The  irregular  openings  in 
the  vein,  the  presence  of  horses,  the  crushed  condition  of  the  quartz  in  many 
parts,  the  presence  of  slickensides  and  of  rolled  pebbles  in  the  clays,  are 
all  conclusive  on  this  point.  Both  to  the  east  and  west  of  the  vein,  too,  the 
country  rock  shows  a  rude  division  into  sheets,  and  along  the  partings 
between  the  plates  evidences  of  movement  are  perceptible,  decreasing  in 
amount  as  the  distance  from  the  vein  increases,  according  to  some  law  not 
directly  inferable.  All  the  evidence  points  to  a  relative  upward  move- 
ment of  the  foot  wall. 


SUMMAEY.  377 

The  question  of  the  character  of  the  contact  surface,  whether  it  is 
a  faulted  sui-face  or  a  continuation  of  a  former  exposure  of  the  east  front 
of  Mount  Davidson,  is  not  to  be  settled  by  mere  inspection.  A  cross-sec- 
tion to  scale  shows  immediately  that  while  the  dip  of  the  lode  is  40°  or 
more,  the  maximum  slope  of  Mount  Davidson  is  about  30°.  This  fact, 
taken  in  connection  with  the  character  of  the  west  wall  where  exposed,  indi- 
cates that  the  surface  is  the  result  of  faulting.  A  natural  surface  sloping 
for  a  long  distance  at  an  angle  of  above  40°,  too,  is  very  unusual.  On  the 
other  hand,  the  coincidence  between  the  contours  of  the  west  wall  and  those 
of  the  exposed  surface  has  been  notorious  from  the  earliest  days  of  mining 
on  the  Lode,  and  it  seems  a  less  violent  supposition  that  the  steep  flank  of 
the  mountain  passes  over  into  the  still  steeper  wall  of  the  vein  than  that 
the  range  has  experienced  an  erosion  modifying  its  angle  from  10°  to  20° 
and  has  still  retained  the  details  of  its  topography  otherwise  unaltered.  It 
is  plain  that  the  elucidation  of  the  faulting  action  on  the  Comstock  is  a 
very  important  structural  problem,  and  that  it  is  most  desirable  to  account 
quantitatively  for  the  results,  as  well  as  to  prove  the  existence  of  a  notable 
dislocation. 

Discussion  of  faulting  under  certain  conditions. The    mOSt     Striking    and    wide- Spread 

evidence  of  the  faulting  is  the  apparent  relative  movement  on  the  contact  sur- 
faces between  more  or  less  regular  sheets  of  the  east  and  west  country  rocks 
for  a  long  distance  in  both  directions  from  the  Lode.  Each  sheet  appears 
to  have  risen  relatively  to  its  eastern  neighbor,  and  to  have  sunk  as  com- 
pared with  the  sheet  adjoining  it  on  the  west.  The  consideration  of  a 
sheet  or  plate  of  rock  under  the  influence  of  friction  of  a  relatively  oppo- 
site character  on  its  two  faces,  therefore,  forms  the  natural  starting  point  for 
an  examination  of  the  observed  conditions.  It  is  shown  in  Chapter  IV.  that 
if  a  country  divided  like  the  Comstock  area  into  parallel  sheets  experiences 
a  dislocation  on  one  of  the  partings  under  a  compressive  strain  equal  at 
each  parting,  a  vertical  cross-section  will  show  a  surface  line  represented 
by  two  logarithmic  equations.  The  discussion  is  also  extended  to  the  case 
in  which  the  compressive  strain  is  not  uniform,  but  varies  proportionally  to 
the  distance  from  the  fault-plane.  This  case  also  results  in  a  logarithmic 
equation  of  a  more  complex  character. 


378  GEOLOGY  OF  THE  COMSTOOK  LODE. 

A  discussion  of  the  logarithmic  equation  as  an  expression  of  faulting 
action  leads  to  some  very  interesting  results,  some  of  which  are  as  follows: 

Where  a  fault  of  the  class  under  discussion  has  occuiTed,  and  where 
the  resulting  surface  has  not  been  obscured  by  deep  erosion,  the  original 
surface  can  be  reconstructed  or  calculated,  and  the  amount  of  dislocation 
determined.     This  is  also  true  where  more  than  one  rock  is  involved. 

Where,  as  is  nearly  always  the  case,  the  movement  on  the  fault-plane 
is  equivalent  to  a  rise  of  the  foot  wall,  the  hanging  wall  seen  in  cross-sec- 
tion will  assume  the  form  of  a  sharp  wedge,  and  this  wedge  will  be  very 
likely  to  yield  to  the  compressive  strain,  and  break  across. 

If  the  movement  of  the  foot  wall  on  the  fault-fissure  were  downward, 
a  surface  line  would  form  which  is  scarcely  ever  met  with  in  nature,  and 
the  inference  is  that  faults  of  this  kind  are  of  extreme  rarity.  This  not 
only  confirms  the  observations  made  in  mines,  but  places  the  fact  on  a  wider 
basis  of  observation. 

If  a  fault,  accompanied  by  compressive  strain,  takes  place  on  a  fissure 
in  otherwise  solid  rock,  the  walls  are  likely  either  to  be  distorted,  if  they 
are  composed  of  flexible  material,  or  to  be  fissured  into  parallel  plates  if  the 
material  is  rigid.  In  the  latter  case  the  sheets  of  rock  will  also  arrange 
themselves  on  logarithmic  curves. 

If  the  intersection  of  a  fault-fissure  with  the  earth's  surface  is  not  a 
straight  line,  but  is  sinuous  or  broken,  the  secondary  fissures  will  be  parallel 
to  the  original  one,  and  in  the  resulting  surface  each  inflection  of  the  trace 
of  the  fissure  on  the  original  surface  concave  toward  the  lower  country  will 
be  represented  on  the  faulted  surface  by  a  ravine,  and  each  inflection  con- 
vex towards  the  lower  country  will  result  on  the  faulted  surface  in  a  ridge. 
There  is  also  a  direct  relation  between  the  contours  of  the  foot  wall  of  such 
a  fissure  and  the  surface  contours.  If  the  original  surface  was  a  horizontal 
plane,  the  surface  contours  will  be  identical  with  the  foot  wall  contours. 

Application  to  the  comstock. — The  thcory,  though  worked  out  independently  of 
the  Comstock,  applies  to  it  with  much  precision.     Equations  can  be  given 
representing  very  closely  the  surface  line  of  a  cross-section,  the  amount  of 
the  fault  can  be  determined,  etc.     It  can  be  shown  that  the  erosion  since  the   » 
beginning  of  the  fault  is  very  slight,  that  the  canons  of  the  range  were  pro- 


SUMMAEY.  V  379 

duced  by  faulting,  and  have  been  only  slightly  modified  by  erosion,  whence 
the  correspondence  of  the  contours  of  the  foot  wall  with  those  of  the  sur- 
face. The  east  fissure  is  a  result  of  the  faulting,  and  the  ore  has  been  de- 
posited since  Washoe  became  a  region  of  insignificant  rainfall.  The  sheeted 
structure  of  the  country  is,  in  all  probability,  due  to  the  fault. 

It  is,  of  course,  most  unlikely  that  the  Comstock  is  the  only  vein  in 
which  the  deposition  of  ore  is  recent  and  has  been  accompanied  by  faulting, 
and  a  repetition  of  a  part  of  the  conclusions  as  to  the  occurrence  of  veins 
in  such  cases  may  be  welcome  to  some  readers. 

Application  to  other  veins. — lu  a  locality  modlficd  by  faulting  action,  attended 
by  horizontal  pressure,  the  fact  will  appear  in  the  parallelism  of  the  exposed 
edges  and  faces  of  rock  sheets.  If  erosion  has  not  seriously  modified  the 
surface  resulting  from  the  faulting  action,  the  logarithmic  curve  will  be 
recognizable  to  the  observer  looking  in  the  direction  of  the  strike. 

The  main  cropping  of  the  vein  is  to  be  sought  at  the  point  of  inflec- 
tion of  the  curve,  which  will  be  found  nearly  or  exactly  midway  between 
the  top  and  bottom  of  the  hillside.  One  or  more  secondary  vein-croppings 
should  be  looked  for  below  the  main  cropping,  and  these,  so  far  as  yield  is 
concerned  (but  not  in  regard  to  location  of  claim),  may  prove  even  more 
important  than  the  main  fissure. 

The  dip  of  the  vein  will  be  to  the  same  quarter  as  the  slope  of  the  sur- 
face, but,  of  course,  greater  in  amount.  The  flatter  the  surface  curve  the 
smaller  the  angle  of  dip  will  be.  The  mean  strike  will  be  nearly  or  quite 
at  right  angles  to  the  direction  of  the  spurs  and  ravines  of  the  faulted  area. 

If,  besides  the  movement  of  one  or  the  other  wall  in  the  azimuth  of  the 
dip,  there  has  been  a  dislocation  in  the  direction  of  the  strike,  chimneys 
will  open,  all  of  them  on  the  same  sides  of  the  different  ravines.  Surface 
evidences  will  often  enable  the  prospector  to  determine  on  which  side  the 
chimneys  are  to  be  found.  On  the  barren  sides  evidences  of  crushing  and 
of  closure  of  the  fissures  are  probable. 

The  fissure  is  more  likely  to  have  a  constant  dip  (barring  the  second- 
ary off"shoots)  than  a  constant  strike,  but,  of  course,  irregularities  of  dip, 
like  those  in  strike,  will  result  in  chambers  which  may  be  productive. 

Offshoots  into  the  hanging  wall  may  occur  at  any  depth,  but  none. 


380  GEOLOGY  OP  THE  COMSTOCK  LODE. 

except  those  near  enough  to  the  main  cropping  to  reach  the  surface  where 
it  has  a  very  considerable  slope,  are  likely  to  be  continuous. 

Finally,  it  is  shown  that  the  law  of  land  slips  is  also  capable  of  expres- 
sion by  logarithmic  equations,  and  that  a  large  part  of  the  details  of  the 
topography  of  grassy  hills  is  formed  in  obedience  to  this  law. 

OCOUERENCE  AND  SUCCESSION  OF  BOOKS. 

Succession. — Tho  succcssiou  of  rocks  made  out  in  the  Washoe  District  is 
as  follows:  Granite,  metamorphics,  granular  diorites,  porphyritic  diorites, 
metamorphic  diorites,  quartz-porphyry,  earlier  diabase,  later  diabase  ("black 
dike"),  earlier  hornblende-andesite,  augite-andesite,  later  horablende-ande- 
site,  and  basalt. 

Granite  and  metamorphics. — Granite  occurs  ou  thc  surface  only  in  a  very  lim- 
ited area  near  the  Bed  Jacket  mine,  but  it  is  certain  that  it  has  a  considerable 
underground  development,  for  it  has  been  struck  at  the  Baltimore,  the  Bock 
Island,  and  by  a  tunnel  to  the  southwest  of  the  latter  beyond  the  limits  of 
the  map. 

The  granite  is  overlaid  by  metamorphic  rocks,  which,  Jiowever,  are 
less  metamorphosed  close  to  it  than  at  a  distance  from  it,  and  the  probabil- 
ities are  that  the  sedimentary  strata  were  laid  down  upon  the  massive  rock. 
The  sedimentary  rocks  are  limestones,  crystalline  schists,  and  slate.  They 
are  badly  broken  and  highly  altered,  and  the  search  for  fossils  was  not 
rewarded  by  success;  but  the  general  geology  of  this  part  of  the  Great 
Basin  leaves  little  doubt  that  they  are  Mesozoic.  A  considerable  area 
of  metamorphics  has  been  exposed  in  the  southwest  of  the  region  by  the 
erosion  of  the  overlying  eruptive  masses.  North  and  east  of  Silver  City, 
however,  the  surface  shows  scarcely  any  metamorphics,  while  they  play  a 
large  part  in  the  underground  occurrences  as  far  as  the  Yellow  Jacket.  In 
the  Gold  Hill  mines  black  slates  form  the  foot  wall  of  the  Lode.  They  are 
intensely  colored  with  graphite,  and  often  very  highly  charged  with  pyrite. 
They  are  frequently  mistaken  for  "black  dike,"  but  a  moment's  inspection 
in  a  good  light  shows  their  sedimentary  origin.  The  presence  of  such  car- 
bonaceous rocks  at  greater  depths  would  explain  the  formation  of  hydrogen 


SUMMARY.  381 

sulphide.  There  is  also  an  obscure  occurrence  of  metamorphic  limestones 
in  the  Sierra  Nevada  mine  between  granular  and  micaceous  diorite.  It 
appears  to  be  conformable  to  the  face  of  the  granular  diorite.  The  meta- 
morphics  in  and  aboiit  Gold  Hill  seem  both  to  overlie  and  to  underlie 
diorite,  and  there  is  little  doubt  that  sedimentary  strata  were  present  at  the 
period  of  the  diorite  eruption. 

Between  the  metamorphics  and  the  quartz-porphyry  in  the  southwest 
portion  of  the  area  is  a  considerable  extent  of  metamorphic  diorite.  In 
some  occurrences  this  rock  is  a  distinct  breccia,  and  bears  a  strong  resem- 
blance to  augite-andesites  or  basalts,  while  elsewhere  it  is  extremely  like 
Mount  Davidson  diorite.  Besides  the  surface  occurrences,  it  is  found  par- 
ticularly well  developed  in  the  Silver  Hill  mine. 

Diorites. — The  principal  exposure  of  diorites  is  on  the  west  of  the  Lode 
through  Virginia  City,  but  there  are  several  outlying  occurrences  about 
the  Forman  shaft,  and  again  far  to  the  east  at  the  Lady  Bryan  mine,  which 
show  that  the  underground  development  of  the  rock  is  a  very  extensive  one. 
It  forms  the  foot  wall  of  the  Lode  from  the  Yellow  Jacket  north.  The  diorite 
is  excessively  uneven  in  its  composition,  and  in  almost  any  area  of  a  hun- 
dred feet  square  sevei'al  modifications  are  to  be  found.  This  fact,  taken  in 
connection  with  the  microstructure  of  the  rock,  is  pretty  conclusive  evidence 
that  it  has  never  reached  a  higher  degree  of  fluidity  than  the  plastic  state. 
The  varieties  can  be  roughly  classified  as  granular  diorite,  porphyritic  horn- 
blendic  diorite,  and  porphyritic  micaceous  diorite.  But  intermediate  varie- 
ties are  of  constant  occurrence.  There  seems,  nevertheless,  to  be  a  certain 
amount  of  order  in  the  disposition  of  the  different  varieties.  Mount  David- 
son, from  Bullion  Ravine  to  Spanish  Ravine,  is  almost  altogether  granular, 
but  to  the  north  and  south  of  these  limits  porphyritic  forms  prevail.  In 
the  neighborhood  of  the  Utah  mine  mica  becomes  the  predominant  ferro- 
magnesian  silicate,  and  this  variety  is  also  the  one  which  occurs  in  the 
neighborhood  of  the  Forman  shaft.  How  this  orderly  disposition  of  the 
various  diorites  came  about  is  a  somewhat  obscure  question,  as  a  possible 
answer  to  which  an  hypothesis  is  advanced. 

Diabases. — The  diabaso  appears  but  to  a  very  trifling  extent  upon  the 
surface,  though  it  is  by  no  means  unlikely  that  an  exposure  of  this  rock 


382  GEOLOGY  OP  THE  COMSTOOK  LODE. 

occupied  the  position  now  covered  by  Virginia  City.  Underground  it  is 
extensively  developed  from  the  Overman  to  the  Sierra  Nevada,  and  from  the 
Lode  to  the  Combination  shaft,  as  is  seen  in  the  cross-section  on  the  Sutro 
Tunnel  line,  Atlas-sheet  Vl.  Its  great  importance  is  due  to  the  fact  that  all 
the  important  bodies  of  the  Comstock  have  been  intimately  associated  with 
it,  as  are  many  of  the  other  famous  silver  mines  of  the  world.  This  diabase 
is  of  a  rather  unusual  character,  being  more  than  commonly  porphyritic,  and 
containing  comparatively  little  augite — a  trifle  less  than  twenty  per  cent. 
In  appearance  it  is  often  not  dissimilar  to  the  andesites,  but  the  resemblance 
does  not  extend  to  details.  Almost  the  whole  of  this  diabase  is  greatly 
decomposed,  and  has  hitherto  escaped  recognition  on  that  account.^     > 

Between  the  east-country  diabase  and  the  west  wall  of  the  Comstock 
occurs  a  thin  dike,  which  has  long  been  known  as  "  black  dike."  It  is  only 
in  the  lower  levels  that  fresh  occurrences  of  this  material  have  been  met 
with.  The  "black  dike"  appears  to  be  identical  with  the  Mesozoic  diabases 
of  the  Eastern  States,  from  which  it  is  scarcely  distinguishable  macroscopic- 
ally,  microscopically,  or  chemically.  This  younger  diabase  forms  a  remark- 
ably thin  and  uniform  dike,  nowhere  more  than  a  few  feet  in  thickness, 
extending  from  the  Savage  southward  to  the  Overman,  and  then  branching 
off  to  the  southwest  as  far  as  the  Caledonia  shaft.  This  is  the  only  dike 
known  in  the  District,  excepting  one  of  diorite  in  dionte,  in  spite  of  the 
prevalence  of  eruptive  rocks.  Its  presence  shows  that  the  fissure  on  which 
the  Comstock  Lode  afterwards  formed  was  first  opened  in  pre-Tertiary 
times,  and  its  uniform  thickness  indicates  that  its  intrusion  antedates  any 
considerable  dislocation  on  the  contact.  This  inference  receives  strong 
confirmation  from  the  evidence  alread)'  adduced  that  the  faulting  is  a 
comparatively  recent  phenomenon. 

The  occurrence  of  the  two  diabases  also  goes  a  long  way  toward 
demonstrating  the  nature  of  the  fork  in  the  vein,  which  has  always  been  a 
mysterious  point  in  the  geology  of  the  Lode.  The  prolongation  of  the 
"  black  dike"  beyond  Gold  Hill  is  toward  American  Flat,  whereas  the  older 
diabase  extends  in  the  direction  of  Silver  City. 

Andesites. — Much  the  larger  part  of  the  surface  of  the  District  is  occupied 

'Though  diabase  is  the  most  iraportant  east-country  rock,  it  by  no  means  coincides  in  position 
either  below  ground  or  above  with  the  rocks  which  have  been  regarded  as  propylite. 


STJMMAEY.  383 

by  andesites,  of  which  there  are  three  varieties  distinguishable  both  htho- 
logically  and  geologically.  These  are  a  younger  and  a  later  hornblende- 
andesite,  the  latter  of  which  has  hitherto  been  considered  a  trachyte,  and 
an  augite-andesite  intermediate  in  age.  The  older  hornblende-andesite  has 
in  part  long  been  recognized  as  such,  and  is  deceptive  only  when  highly 
decomposed.  It  occupies  a  belt  immediately  east  of  the  older  diabase  (see 
Atlas-sheet  VII.),  a  large  area  on  the  heights  immediately  west  of  the 
diorites,  and  a  considerable  area  at  and  north  of  Silver  City.  The  latter 
occurrence  is  noteworthy  for  the  unusual  size  of  the  hornblendes,  which 
are  sometimes  several  inches  in  length.  The  augite-andesite  occupies  a 
second  belt  of  country  east  of  the  Lode  and  beyond  the  earlier  hornblende- 
andesite,  and  is  also  extensively  developed  to  the  north  and  south  of  the 
diorite.  The  Forman  shaft  penetrates  1,200  feet  of  this  rock  before  pass- 
ing into  the  hornblende-andesite. 

The  reasons  are  given  elsewhere  for  considering  the  rock  heretofore 
regarded  as  trachyte  to  be  an  andesite.  Its  roughness  and  softness,  its  red 
and  purple  colors  and  large  glassy  feldspars  made  the  mistake  an  easy  one. 
The  Flowery  Range,  the  Sugar  Loaf,  Mount  Emma,  and  Mount  Rose,  are 
all  of  this  rock,  which  also  occurs  in  two  little  patches  close  to  the  Sierra 
Nevada  mine.  These  latter  have  been  cut  off  from  the  quarr}^  above  the 
Utah  by  the  erosion  of  Seven-Mile  Canon.  The  patches  of  rock  near  the 
Combination  shaft  and  the  new  Yellow  Jacket  which  have  sometimes  been 
regarded  as  trachyte  are  merely  decomposed  older  hornblende-andesite. 

Basalt. — The  occurrence  of  basalt  is  exceedingly  limited,  and  is  confined 
within  the  area  of  the  map  to  two  small  localities,  one  at  Silver  City  and 
the  other  a  mile  west  of  it.     It  is  a  fine,  fresh,  and  typical  rock. 

Area  of  decomposition. — Thc  aroa  of  most  profouud  decomposition  is  shown 
as  nearly  as  may  be  on  the  sketch  map.  Fig.  1 .  The  amount  of  decomposi- 
tion increases  with  depth.  The  period  at  which  it  was  produced  is  almost 
certainly  the  same  as  that  of  the  faulting  action  and  the  deposition  of  ore. 
It  cannot  have  been  earlier  than  the  eruption  of  the  later  hornblende-ande- 
site, and  was  more  probably  posterior  to  it.  There  is  no  indication  of  a 
connection  between  the  basalt  eruption  and  the  solfataric  action,  and  it  is 
not  improbable  that  the  latter,  though  of  volcanic  origin,  was  independent 
of  any  eruption  of  lava. 


384  GEOLOGY  OF  THE  COMSTOCK  LODE. 


CHEMISTRY. 

The  chemical  history  of  the  Comstock  is  no  doubt  a  very  complex 
one,  nor  are  there  by  any  means  sufficient  data  to  trace  it  in  detail.  AH 
that  can  be  attempted  here  is  to  show  that  the  results  observed  might 
naturally  follow  from  highly  probable  causes. 

The  decomposition  of  the  rocks  shows  three  important  features — the 
formation  of  pyrite  from  the  bisilicates  and  mica,  the  decomposition  of  the 
ferro-magnesian  silicates  into  chlorite,  which  is  in  part  further  altered  to 
epidote,  and  a  partial  change  of  the  feldspar. 

Decomposition  of  the  Fe.-Mg.  sUicates. — The  pyrite  appcars  to  have  formed  at  the 
expense  of  the  bisilicates  or  mica.  The  really  fresh  rocks  contain  no  pyrite, 
but  minute  crystals  often  occur  in  or  are  attached  to  partially  decomposed 
bisilicates.  Sometimes  distinct  pseudomorphs  of  pyrite  after  augite  or 
hornblende  are  visible,  but  this  is  not  common,  because  the  average  size  of 
the  pyrite  crystals  is  about  one-half  that  of  their  hosts.  A  macroscopical 
comparison,  too,  of  series  of  rocks  increasingly  decomposed  shows  that  the 
pyrite  is  apparently  associated  with  the  ferro-magnesian  silicates,  and  in 
extreme  cases  replaces  them  with  an  entire  correspondence  of  distribution, 
so  that  the  cumulative  evidence  is  all  in  one  direction.  It  is  well  known 
that  ferrous  silicates  in  contact  with  waters  charged  with  hydrogen  sulphide 
produce  pyrite. 

The  transformation  of  the  bisilicates  and  mica  to  chlorite  is  a  familiar 
fact,  and  the  general  character  of  the  change  is  not  obscure,  though  its 
details  are  far  from  clear.  It  must  be  accompanied  by  a  separation  of 
all  the  lime,  and  of  much  of  the  silica  and  magnesia.  It  probably  took 
place  for  the  most  pai-t  in  the  absence  of  free  oxygen. 

Epidote  is  very  common  on  the  surface,  while  under  ground  it  seems 
rare  and  confined  to  the  neighborhood  of  fissures.  The  conversion  of 
chlorite  to  epidote  must  be  accompanied  by  a  substitution  of  lime  for  mag- 
nesia, and  by  the  conversion  of  ferrous  to  ferric  oxide  It  might  very 
readily  occur  in  the  presence  of  solutions  containing  carbonic  acid  and  free 
oxygen,  or  when  surface  waters  mingled  with  waters  lising  from  lower 


SUMMAEY.  385 

levels,  for  epidote  is  far  less  soluble  than  chloritg,  and  under  these  circum- 
stances would  form  in  obedience  to  the  general  law  of  precipitation.  Its 
occurrence  is  usually  compatible  with  this  supposition,  but  it  is  not  so  deci- 
sive as  to  warrant  a  positive  assertion  that  the  conditions  of  its  formation 
are  those  indicated. 

Decomposition  of  feldspars. — The  tricliuic  feMspars  of  the  Washoe  District 
retain  their  optical  properties  in  a  recognizable  form  much  longer  than  the 
ferro-magnesian  silicates.  Among  the  mine  rocks  it  is  very  rarely  that  bisili- 
cates  or  mica  occur  undecomposed,  but  it  is  the  exception  when  a  slide  of  a 
tolerably  hard  rock  does  not  show  recognizable  feldspars.  When  the  feld- 
spars are  altered  they  are  replaced  by  an  aggregate  of  polarizing  grains, 
which  appear  to  be  quartz  and  calcite  with  some  opaque  particles,  but  with 
no  transparent  amorphous  material.  Kaolin  could  hardly  be  present  in 
large  quantities  without  being  recognized  microscopically.  The  analyses 
of  the  clays,  too,  show  that  when  allowance  is  made  for  the  presence  of 
hydrovis  chlorite  there  is  not  enough  water  to  correspond  to  any  large 
percentage  of  kaolin.  In  fact  the  analyses  of  the  clays  so  exactly  cor- 
respond to  the  composition  of  the  firm  rocks  that  the  great  masses  of  clay 
evidently  represent  only  equal  volumes  of  disintegrated  rock.  On  the 
whole,  therefore,  it  appears  improbable  that  there  has  been  any  great  amount 
of  kaolinization  in  the  Washoe  Distkict. 

Lateral-secretion  theory. — As  is  wcll  kuowu,  Prof.  F.  Saudbergcr  has  very 
ably  maintained  what  is  known  as  the  lateral-secretion  theory  of  ore  depos- 
its. With  a  view  to  testing  the  probabilities  of  this  theory,  with  reference 
to  the  CoMSTOCK,  the  rocks  of  the  District  have  been  assayed  with  all  pos- 
sible precaution.  The  principal  rocks  containing  precious  metals  were  also 
separated  by  Thoulet's  method,  and  the  precious  metals  traced  to  their 
mineralogical  source.  The  results  of  this  investigation  show  many  inter- 
esting facts,  among  which  are  the  following:  The  diabase  shows  a  note- 
worthy contents  in  the  precious  metals,  most  of  which  is  found  in  the  augite; 
the  decomposed  diabase  contains  about  half  as  much  of  these  metals  as  the 
fresh  rock;  the  relative  quantities  of  gold  and  silver  in  the  fresh  and  decom- 
posed diabase  correspond  fairly  well  with  the  known  composition  of  the 

CoMSTOCK    bullion;  and  the  quantity  of  precious  metals  which  has  been 
25  c  L 


386  GEOLOGY  OF  THE  COMSTOCK  LODE. 

leached  out  of  the  diabase  is  comparable  with  that  which  the  Lode  must 
have  contained  at  its  discovery.  There  are  also  relations  between  the 
inclosing  rocks  and  the  ore  deposits  not  found  in  contact  with  diabase. 

The  gangue  of  the  Comstock  is  almost  exclusively  quartz,  though  cal- 
cite  also  occurs  in  limited  areas.  The  ore  minerals  elude  investigation  for 
the  most  part  because  they  are  so  finely  disseminated  as  merely  to  stain 
the  quartz,  but  it  is  fairly  certain  that  they  are  principally  argentite,  and 
native  silver  and  gold,  accompanied  in  some  cases  by  sulph-antimonides, 
etc.  The  chloride  has  rarely  been  identified.  Where  ore  is  found  in  dio- 
rite,  or  in  contact  with  it,  it  is  usually  of  low  grade,  and  its  value  is  chiefly 
in  gold.  The  notably  productive  ore  bodies  have  been  found  in  contact 
with  diabase,  and  they  have  yielded  by  weight  about  twenty  times  as  much 
silver  as  gold. 

Reagents. — It  would  perhaps  be  legitimate  to  infer  from  the  chemical 
phenomena  enumerated  and  the  .association  of  minerals  that  waters  charged 
with  carbonic  acid  and  hydrogen  sulphide  had  played  a  considerable  part 
on  the  Comstock.  This  is  not,  however,  a  mere  inference,  for  an  advance 
boring  on  the  3,000-foot  level  of  the  Yellow  Jacket  struck  a  powerful  stream 
of  water  at  3,080  feet  (in  the  west  country),  which  was  heavily  charged  with 
h3-drogen  sulphide  and  had  a  temperature  of  170°  F ,  and  there  is  equal 
evidence  of  the  presence  of  carbonic  acid  in  the  water  of  the  lower  levels. 
A  spring  on  the  2,700-foot  level  of  the  Yellow  Jacket,  which  showed  a  tem- 
perature of  above  1.50°  F.,  was  found  to  be  depositing  a  sinter  largely  com- 
posed of  carbonates. 

Baron  v.  Richthofen  was  of  opinion  that  fluorine  and  chlorine  had 
played  a  large  part  in  the  ore  deposition  on  the  Comstock,  and  that  this 
is  possible  cannot  be  denied;  but,  on  the  other  hand,  it  is  plain  that  most 
of  the  phenomena  are  sufficiently  accounted  for  on  the  supposition  that  the 
agents  have  been  merely  solutions  of  carbonic  and  hydrosulphuric  acids. 
These  reagents  will  attack  the  bisllicates  and  feldspars.  The  result  would 
be  carbonates  and  sulphides  of  metals,  earths  and  alkalies,  and  free  quartz ; 
but  quartz  and  the  sulphides  of  the  metals  are  soluble  in  solutions  of  car- 
bonates and  sulphides  of  the  earths  and  alkalies,  and  the  essential  constitu- 
ents of  the  ore  might,  therefore,  readily  be  conveyed  to  openings  in  the 


SUMMAEY.  387 

vein,  where  they  would  have  been  deposited  on  relief  of  pressure  and  dimi- 
nution of  temperature.  It  is  by  no  means  unlikely  that,  as  at  Steamboat 
Springs,  evaporation  aided  in  inducing  precipitation 

Substitution.  —  It  has  been  claimed  that  the  ore  and  qviartz  have  been 
deposited  by  substitution  for  masses  of  country  rock.  This  hypothesis  is 
exceedingly  doubtful  on  chemical  grounds,  but  there  is  also  at  least  one  in- 
superable physical  objection  to  it.  In  all  processes  involving  the  solution 
of  angular  bodies  it  is  a  matter  of  common  observation  that  points  and 
corners,  which  expose  a  greater  surface  than  planes,  are  first  attacked;  conse- 
quently masses  exposed  to  solution,  substitution,  weathering,  and  the  like, 
always  tend  to  spheroidal  forms.  Now,  nothing  is  more  common  than  to 
find  masses  of  country  rock  included  in  the  ore-bearing  quartz.  These 
masses,  in  all  cases  which  have  come  under  my  observation,  are  angular 
fragments,  in  form  precisely  such  as  result  from  a  fresh  fracture;  not  a 
single  instance  has  been  observed  in  which  a  spheroidal  rock  was  sur- 
rounded by  more  and  more  polyhedral  concentric  shells  of  quartz  and  ore. 


HEAT  PHENOMENA  OF  THE  LODE. 

High  temperatures  met. OuC    of     thc    faUlOUS    pCCuliaritieS    of     the    COMSTOCK 

Lode  is  the  abnormally  high  temperature  which  prevails  in  and  near  it. 
This  manifested  itself  in  the  upper  levels,  and  has  increased  with  the  depth. 
The  present  workings  are  intensely  hot.  The  water  which  flooded  the  lower 
levels  of  the  Gold  Hill  mines  dui-ing  the  winter  of  1880-1881  had  a  tem- 
perature of  170°  F.  This  water  will  cook  food,  and  will  destroy  the  human 
epidermis,  so  that  a  partial  immersion  in  it  is  certain  death.  The  air  in  the 
lower  levels  more  or.  less  nearly  approaches  the  temperature  of  the  water 
according  to  the  amount  of  ventilation.  The  rapidity  of  the  ventilation 
attained  in  the  mines  is  something  unknown  elsewhere,  yet  deaths  in  venti- 
lated workings  from  heat  alone  are  common,  and  there  are  drifts  which, 
without  ventilation,  the  most  seasoned  miner  cannot  enter  for  a  moment. 
Except  where  circulation  of  air  is  most  rapid,  and  in  localities  not  far 
removed  from  downcast  shafts,  the  air  is  very  nearly  saturated  with  moist- 


388  GEOLOGY  OF  THE  COMSTOOK  LODE, 

lire.  It  is  a  serious  question  how  far  down  it  will  be  possible  to  pusli  the 
mines  in  spite  of  the  terrific  heat. 

The  origin  of  the  high  temperature  of  the  Comstock  has  been  sought 
in  the  kaolinization  of  the  feldspar  contained  in  the  country  rock  and  in 
residual  volcanic  activity.^ 

Kaolinization  hypothesis. — The  theory  that  kaoliuizatiou  is  the  cause  of  the 
heat  appears  to  rest  upon  two  positive  grounds — that  the  solidification 
of  water  liberates  heat,  and  that  flooded  drifts  have  been  observed  to  grow 
hotter.  It  is  also  claimed  in  favor  of  the  kaolinization  hypothesis  by  its 
author  that  there  is  no  evidence  of  an}'  other  chemical  action  proceeding 
with  sufiicient  activity  to  afford  an  explanation,  and  that  the  retention  of 
igneous  heat  in  the  rocks  is  a  sheer  impossibility,  while  the  hypothesis  that 
the  heat  is  conveyed  from  some  deep-seated  source  to  the  mines  by  means 
of  currents  of  heated  water  is  characterized  as  somewhat  violent  and  as 
unnecessary. 

So  far  as  I  am  aware,  there  are  no  theoretical  grounds  upon  which 
the  heat  involved  in  kaolinization  can  be  estimated.  The  decomposi- 
tion of  feldspar  into  kaolin  and  other  products  (supposing  kaolin  to  result 
from  the  decomposition  of  plagioclase)  involves  several  processes,  of 
which  some  are  more  likely  to  absorb  than  to  liberate  heat.  But  sup- 
posing an  anhydrous  aluminium  silicate  formed  without  loss  of  heat,  the 
thermal  results  of  its  combination  with  water  are  by  no  means  certain. 
Were  the  water  contained  in  kaolin  not  water  of  hydration,  but  chemically 
combined,  it  would  be  possible  from  known  experiments  to  compute  approxi- 
mately the  heat  which  would  be  produced.  It  is  shown  in  Chapter  VII. 
that  the  corresponding  temperature  would  be  so  high  as  to  be  utterly  at 
variance  with  known  facts.  The  water  is  therefore  the  water  of  hydration. 
Of  the  heat  involved  in  the  hydration  of  salts  we  know  that  it  is  usually 
small,  that  it  is  sometimes  negative,  and  that  the  different  molecules  of  water 
combine  with  differing  amounts  of  energy,  but  of  the  heat  of  hydration  of 
kaolin  we  know  nothing. 

With  a  view  to  testing  the  theory  of  kaolinization  as  far  as  possible, 
Dr.   Barus,   at   my   request,   undertook    some    very    delicate    experiments 

'  Friction  iind  the  oxidaliou  of  pyrite  have  also  been  suggested,  but  have  not  been  seriously  advo- 
cated. 


StlMMARY.  3g9 

presently  to  be  described.  The  result  of  these  experiments,  in  a  word, 
was  that  finely  divided,  almost  fresh  east-country  diabase,  exposed  to  the 
temperature  of  boiling  water  and  the  action  of  saturated  aqueous  vapor  for 
a  week  at  a  time,  and  for  several  weeks  in  succession,  showed  no  rise  of 
temperature  perceptible  with  an  apparatus  delicate  to  the  i-„Vu  of  a  degree  C. 

It  is  by  no  means  certain  that  kaolinization  was  effected  by  these  exper- 
iments. The  particles  of  rock  were  indeed  coated  with  a  white  powdery 
substance,  but  this  was  probably  the  residuum  of  the  evaporated  water. 
It  is  still  possible  that,  when  kaolinization  occurs,  heat  is  liberated.  It 
is  also  possible  that  at  temperatures  above  the  boiling  point  and  pres- 
sures greatly  exceeding  760°"°,  feldspars  are  kaolinized,  but  it  appears 
no  longer  reasonable  to  ascribe  the  heating  of  drifts,  which  are  at  nearly 
normal  pressure,  to  the  reaction  on  the  rocks  of"  water  below  the  boiling 
point.  The  scene  of  active  and  heat-producing  kaolinization,  if  it  exists  at 
all,  must,  therefore,  be  at  remote  depths.  As  was  explained  in  a  previous 
paragraph,  the  present  examination  has  not  i-esulted  in  tracing  any  consid- 
erable amount  of  kaolinization  on  theCoMSTocK;  while,  had  the  heat  of  the 
Lode  been  maintained  ever  since  its  formation  at  the  expense  of  the  feld- 
spars, but  little  undecomposed  feldspar  could  now  remain.  In  short,  while 
it  cannot  be  demonstrated  that  the  heat  of  the  Comstock  is  not  due  to  the 
prevalence,  at  unknown  depths  and  pressures,  of  a  chemical  change  of  un- 
known thermal  relations,  I  have  failed  to  find  any  proof  that  it  is  due  to 
kaolinization. 

soifataric  action. — Of  tlic  origiii  of  tlic  heat  of  solfataras  not  very  much  is 
known;  yet,  as  they  commonly  occur  either  as  an  accompaniment  of  vol- 
canic activity,  or  in  regions  characterized  by  the  strongest  evidences  of  past 
volcanic  activity,  it  is  usual  and  seems  rational  to  connect  them  as  cause 
and  eff"ect,  or  as  different  effects  of  a  common  cause.  There  seems  to  be  no 
special  opportunity  on  tlie  Comstock  for  an  elucidation  of  the  wliole  theory 
of  vulcanism,  but  considerable  grounds  for  connecting  the  heat  there  mani- 
fested with  that  chain  of  phenomena. 

That  soifataric  action,  as  commonly  understood,  once  existed  on  the 
Comstock  is  certain.  That  the  time  at  which  the  Lode  was  charged  with 
ore  is  not  immeasurably  removed  from  the  present,  seems  to  be  demon- 


390  GEOLOGY  OF  THE  COMSTOCK  LODE. 

strated  by  the  trifling  character  of  the  erosion  which  has  since  taken  place. 
The  water  entering  at  the  bottom  of  the  new  Yellow  Jacket  shaft  in  the  win- 
ter of  1880-81,  at  a  temperature  of  170°  F.,  was  highly  charged  with 
hydrogen  sulphide.  The  Steamboat  Springs,  only  a  few  miles  west  of  the 
CoMSTOCK,  lie  in  a  north  and  south  line  like  the  Comstock,  close  to  the  con- 
tact of  ancient  massive  rocks  and  andesites.  Some  of  them  are  boiling  hot, 
are  charged  with  solfataric  gases,  and  are  now  depositing  cinnabar  and  silica 
as  at  the  time  of  Mr.  Phillips's  visit  many  years  ago.  There  is  much  evi- 
dence in  the  structure  of  the  country  and  in  the  relations  of  the  fresh  rocks  to 
the  decomposed  masses  that  alteration  was  effected  by  rising  waters,  and 
the  chemical  changes  traced  are  such  as  could  have  been  effected  only  by 
vast  quantities  of  soluble  sulphides  and  carbonic  acid,  which  could  hardly 
have  been  produced  on  the  necessary  scale  except  by  the  aid  of  heat.  A 
deep-seated  source  of  heat,  therefore,  probably  gave  rise  to  the  decomposi- 
tion, and  the  conditions  point  to  vulcanism  as  its  source. 

Source  of  the  waters. — Thc  flood  of  waters  stlU  rcqulrcs  explanation,  and  an 
hypothesis  is  suggested  to  account  for  it.  No  meteorological  station  exists 
at  Virginia  City,  but  the  rainfall  is  so  small  that  the  country  is  a  sage-brush 
desert,  and  the  precipitation  is  insufficient  to  account  for  the  water  met  with 
on  the  Lode.  The  main  influx  of  water,  and  especially  of  hot  water,  is 
from  the  west  wall,  and  when  encountered  it  is  found  under  a  head  often 
of  several  hundred  feet.  Between  the  Comstock  and  the  main  rana^e  of  the 
Sierra  Nevada,  the  whole  country  is  covered  by  massive  rocks,  principally 
andesites,  with  occasional  croppings  of  granite.  The  general  structure  of 
the  country,  and  the  exposures  of  sedimentary  rocks  in  the  mines,  lead  to 
the  supposition  that  the  underlying  strata  dip  eastward,  and  the  inference 
is  that  the  Comstock  fissure  taps  water-ways  leading  from  the  crests  of  the 
great  range.  If  the  heat  is  conveyed  to  the  Lode  by  waters  from  great 
depths,  the  variations  in  temperature  are  readily  explained.  The  distribu- 
tion of  the  heated  waters  would  be  determined  by  the  presence  of  cracks, 
fissures,  and  clay-seams,  and  the  uniformity  of  distribution  of  heat  would 
further  be  disturbed,  even  at  considerable  distances  from  the  surface,  by  the 
infiltration  of  surface  water.  One  published  observation,  which  is  impor- 
tant in  this  connection,  is  that  a  large  proportion  of  the  rocks  in  the  Vir 


SUMMARY.  391 

ginia  mines  are  dry.  This  is  very  true  in  the  sense  in  which  "dry"  is  used  in 
mining,  i.  e.,  there  are  many  places  where  water  does  not  drip  from  the 
walls,  but  the  present  examination  has  failed  to  reveal  rocks  which  are  not 
moist ;  indeed,  the  occurrence  of  really  desiccated  rock  thousands  of  feet 
below  the  surface,  near  vast  quantities  of  water,  would  disprove  the  gen- 
eralization of  the  perviousness  of  rocks,  which  is  one  of  the  best  established 
in  geology.  Unless,  therefore,  very  strong  proof  to  the  contrary  can  be 
adduced,  the  conduction  of  heat  on  the  Comstock  must  be  considered  as 
taking  place  in  moist  rock. 

Discussion  of  the  thermometric  observations. Thc  rclatlon   of   the   tCmperatUrC   tO  the 

depth  from  the  surface  is  evidently  one  of  great  interest,  but  not  entirely 
simple.  If  the  rock  were  wholly  uniform  in  character  and  unfissured,  the 
relation  of  temperature  to  depth  would  be  wholly  regular  and  would  be 
represented  by  a  curvilinear  locus.  As  the  source  of  the  heat  was  ap- 
proached the  rate  at  which  the  temperature  rose  would  rapidly  increase, 
and  under  the  ideal  conditions  supposed,  it  would  be  possible  to  deduce  the 
constants  of  the  equation  and  to  calculate  the  position  of  the  source  of  heat. 
But  unless  the  source  of  heat  were  so  close  to  the  surface  that  the  errors 
introduced  by  the  presence  of  fissures,  the  lack  of  homogeneity  of  the  rock, 
and  the  percolation  of  surface  water  were  insignificant  in  comparison  with 
the  rate  of  increase  of  the  temperature,  such  a  calculation  would  not  be 
possible.  A  careful  record  of  temperatures  has  been  kept  at  three  of  the 
newer  shafts  to  a  depth  of  above  2,000  feet.  On  plotting  these  temperatures 
as  ordinates  and  the  depths  as  abscissae  no  indication  of  regular  curvature 
appears,  being  wholly  obscured  by  the  fluctuations  due  to  the  disturbing 
causes  mentioned.  In  other  words,  there  is  as  yet  nothing  in  the  observa- 
tions to  show  any  but  local  divergences  from  a  strict  proportionality  between 
depth  and  temperature.  The  source  of  heat  must,  consequently,  lie  at  a 
very  great  distance  from  the  surface  as  compared  with  the  depth  yet  reached, 
and  the  curve  is  to  be  regarded  as  still  sensibly  coincident  with  its  tangent. 
In  order  to  eliminate  the  fluctuations  of  temperatui-e  as  far  as  possible 
Mr.  Reade  and  Dr.  Barus  have  computed  the  observations  madci  at  the 
Forman,  Combination,  and  new  Yellow  Jacket  shafts  by  the  method  of  least 


392  GEOLOGY  OF  THE  COMSTOCK  LODE. 

squares,  and  also,  for  comparison  with  them,  the  observations  of  Mr.  J.  A. 
Phillips  at  the  Rose  Bridge  Colliery. 

Chapter  VII.  contains  the  details  for  these  localities  and  for  the  famous 
deep  boring  at  Sperenberg,  near  Berlin.  Here  it  is  sufficient  to  state  that 
on  the  CoMSTOCK  the  temperature  of  the  rock  rises  about  3°  F.  for  every 
additional  depth  of  100  feet,  or  about  twice  as  fast  as  in  ordinary  localities; 
and  that  boiling  water  will  probably  reach  the  workings  at  some  point  not 
long  after  the  4,000-foot  level  is  passed. 

Observations  have  also  been  made  in  the  Sutro  Tunnel,  and  these  when 
plotted  give  a  very  remarkable  result,  for  the  curve  shows  that  the  tempera- 
ture rises  in  a  geometric  ratio  as  the  Lode  is  approached.  This  is  capable 
of  no  other  explanation  than  that  the  east  country  is  heated  from  a  plane 
in  the  immediate  neighborhood  of  the  Lode.  Combined  with  the  results 
obtained  from  the  shafts  this  curve,  without  any  reference  to  geological 
reasoning,  indicates  that  the  source  of  heat  is  at  a  vast  depth  compared 
with  that  of  the  mines,  and  that  the  heat  is  communicated  upward  along  or 
near  the  fissure,  and  thence  to  the  country  rock  by  conduction. 


THE  LODE. 

General  character  of  the  vein. The     COudition    of   the    LODE     duriug    the    period 

in  which  the  field  work  for  this  report  was  done  was  not  what  could  have  been 
wished,  for  almost  the  only  ore  in  sight  was  the  remnants  of  the  great  bonanza 
of  the  Consolidated  Virginia  and  the  California,  and  the  accessible  exposures 
of  the  vein  were  meager  and  unsatisfactory.  The  study  of  the  Comstock 
was  thus  necessarily  directed  to  the  conditions  of  its  occurrence  rather  than 
to  details  of  vein  structure. 

A  glance  at  the'  surface  map  shows  that  the  Lode  is  a  long  and  wide  belt 
of  vein-matter  ramifying  at  each  end  into  divergent  branches,'  and  the  cross- 
section  exhibits  a  remarkably  regular  foot  wall  dipping  to  the  east  at  an 

'  TBe  scale  of  the  surface  map  is  not  large  enough  to  permit  all  of  the  minor  fluctuations  of  the 
walls  to  be  shown,  nor  are  the  mine  maps  sufficiently  complete  to  furnish  data  for  a  full  exhibition  of 
these  irregularities  on  a  larger  scale.  For  more  detail  the  reader  is  referred  to  Mr.  King's  section  on  the 
331-foot  level  of  the  Virginia  mines. 


SUMMAEY.  393 

angle  of  from  33°  to  45°.  Near  the  top,  in  most  of  the  sections,  one  or  more 
secondary  fissures  diverge  from  the  main  Lode  and  penetrate  the  east  wall, 
thus  cutting  off  a  body  of  country  rock,  or  "horse,"  which  is  approximately 
triang-ular  in  cross-section.  This  horse  is  of  variable  vertical  and  horizontal 
dimensions,  and  often  divided  by  sheets  of  clay  or  quartz.  Below  the  horse 
the  vein  is  for  the  most  part  narrow. 

The  walls. — The  hanging  wall  of  the  Lode  between  the  points  at  which 
it  branches  is  older  diabase,  which  also  extends  some  distance  on  the  south- 
east branch  towards  the  Justice,  and  towards  the  Scorpion  on  the  northeast 
branch.  Its  limits  in  the  latter  direction  are  unknown.  Almost  all  of  this 
diabase  is  in  an  advanced  stage  of  decomposition.  The  foot  wall  of  the 
main  fissure  is  granular  diorite,  except  in  Gold  Hill,  where  this  rock  is  re- 
placed by  metamorphic  slates,  and  is  much  less  decomposed  than  the  hang- 
ing wall.  The  northern  and  southern  branches  of  the  vein  pass  through  or 
along  the  contacts  of  various  older  rocks.  The  black  dike  or  younger  dia- 
base appears  in  the  Savage  and  Hale  &  Norcross,  but  not  to  the  north  of  these 
mines,  and  has  been  followed  on  or  near  the  foot  wall  to  the  fork  at  the 
Overman,  and  thence  in  a  southwesterly  direction  toward  American  Flat. 
It' is  the  behavior  of  the  two  diabases  which  has  given  rise  to  this  fork,  the 
older  diabase  forming  the  hanging  wall  of  the  easterly  branch  for  some  dis- 
tance from  its  origin,  while  the  narrow  dike  of  the  younger  variety  marks  the 
course  of  the  westerly  vein.  To  the  north  also  there  are  indications  that 
the  direction  of  the  branches  was  predetermined ;  the  northeasterly  one  by 
the  contact  between  diabase  and  diorite,  and  that  which  has  been  explored 
in  the  Sierra  Nevada  and  Utah  mines  by  the  presence  of  metamorphic  rocks 
and  some  intrusive  stringers  of  diabase. 

Contents  of  the  vein. — Thc  contcuts  of  thc  veiu  is  simple  on  the  whole. 
Besides  fragments  of  country  rock,  practically  the  only  gangue  which  it 
contains  is. quartz;  though  calcite  occurs  In  insignificant  quantities  in  the 
main  Lode,  and  is  the  prevalent  mineral  in  the  Justice.  The  principal  ore 
is  argentite,  accompanied  by  gold,  probably  in  a  free  state,  though  sulphur 
salts  occasionally  form  rich  stringers  and  pockets.  The  distribution  of  ore 
is  very  variable.  That  associated  with  the  diorite  carries  a  little  gold  and 
almost  no  silver,  while  that  associated  with  diabase  is  regarded  as  a  silver 


394  GEOLOGY  OF  THE  COMSTOCK  LODE. 

ore,  though  nearly  half  its  value  is  usually  in  gold.  The  proportion  of  the 
two  metals  varies  greatly  in  different  portions  of  the  Lode  and  even  in  the 
same  ore  body.  It  is  probable  that  the  Comstock  contains  but  little  quartz 
which  is  wholly  barren;  while,  as  is  usual  in  silver  veins,  it  is  only  in  cer- 
tain spots  that  the  tenor  reaches  a  point  at  which  extraction  is  profitable. 
These  concentrations,  or  "bonanzas,"  usually  occur  in  masses  of  quartz  of 
lower  grade,  and  large  bodies  of  quartz  usually  contain  "bonanzas"  when 
they  are  associated  with  the  diabase,  though  to  this  rule  there  are  exceptions. 
The  Justice  bonanza  is  the  only  one  of  any  moment  which  is  not  associated 
with  that  rock.  The  quartz  is  in  great  part  in  a  highly  crushed  condition, 
resembling  nothing  so  much  as  ordinary  commercial  salt.  When  the  fine 
dust  from  such  masses  is  examined  under  the  microscope  in  polarized  light, 
it  is  immediately  seen  to  consist  of  fi-agments  of  quartz  crystals;  the  larger 
particles  can  be  shown  to  have  the  same  origin  by  direct  examination. 
Very  solid  quartz  bodies  are  also  met  with  in  certain  positions. 

Crushing  action. — The  prcseucc  of  faults  on  the  Comstock  is  abundantly 
proved,  as  has  already  been  shown.  The  secondary  fissures  form  one  evi- 
dence of  such  a  movement,  and  as  a  large  portion  of  the  ore  occupies  the 
openings  between  the  great  horse  and  the  east  country,  it  is  plain  that  the 
deposition  of  ore  was  preceded  by  faulting.  The  only  movement  which  can 
have  crushed  the  quartz  must  also  have  been  in  the  nature  of  a  fault,  and 
some  of  the  bonanzas  show  a  parallelism  in  the  lines  of  dynamical  action 
to  the  dip  of  the  Lodk.  It  is  not  probable  that  the  solid  masses  of  quartz 
were  formed  at  a  later  date  than  those  now  found  in  a  crushed  condition, 
for  it  appears  to  be  onl}^  when  the  quartz  has  been  deposited  in  sheets  par- 
allel to  the  west  wall  that  it  has  escaped  comminution.  Certain  stringers  of 
rich  ore  in  the  bonanzas  have  seemed  to  possess  great  solidity,  and  may 
possibly  have  been  formed  after  the  final  cessation  of  movement;  other- 
wise the  entire  period  of  quartz  deposition  seems  to  have  been  embraced  by 
that  of  the  faulting  movement.  It  is  much  more  probable  that  the  total 
fault  was  accomplished  by  a  great  number  of  small  slips  in  the  same  sense 
than  that  one  large  throw  preceded  and  another  followed  the  filling  of  the 
vein. 

Clays. — The  clays  of  the  Comstock  are  not  largely  composed  of  kaolin, 


SUMMARY.  395 

but  represent  sheets  of  rock  triturated  and  decomposed  without  any  great 
translocation  of  material.  This  fact  is  determined  chemically,  but  confirmed 
by  the  relation  of  the  claj's  to  the  faulted  structure;  for  while  they  are  excess- 
ively abundant  in  and  near  the  secondary  fissure  where  the  influence  of  the 
surface  interfered  with  the  development  of  the  regular  system  of  fissures 
found  in  the  lower  levels,  they  are  comparatively  rare  and  thin  below  the 
bottom  of  the  great  horse. 

Infrequency  of  lenticular  openings. Thc     SCCtioUS     shoW    that     the     loWCr     pOrtioUS 

of  the  Lode,  considering  its  enormous  scale,  are  narrow  and  remarkable 
for  the  absence  of  the  lenticular  openings  which  frequently  characterize 
faulted  veins.  If  the  hypothesis  developed  under  the  head  of  the  structural 
results  of  faulting  is  correct,  this  peculiarity  is  almost  a  necessary  conse- 
quence of  the  conditions  under  which  the  Comstock  formed,  for  the  slip  of 
the  actual  walls  of  the  vein  is  on  that  theory  only  the  relative  movement  of 
two  successive  sheets,  and  if  these  are  assumed  to  be  twenty -five  feet  thick, 
it  would  not  amount  to  above  a  hundred  feet.  The  intensity  of  the  fault- 
ing action  was  less  toward  the  ends  of  the  Lode  than  near  the  middle, 
the  force  being  distributed  over  a  wide  area  by  the  branching,  and  probably 
also  to  some  extent  by  numerous  east-and-west  fractures,  singly  of  small 
extent.  The  south  end  of  the  main  Lode  seems  to  have  been  less  forcibly 
faulted  than  the  north  end  This  is  partly  ascribable  to  the  character  of  the 
foot  wall,  stratified  rocks  being  less  rigid  than  massive  ones,  and  partly  to 
the  fact  that  the  dip  is  about  10°  less 

Character  of  the  spaces  occupied  by  bonanzas. TllC   evideUCe  appCarS     COnclusive  that 

the  ore  bodies  occupy  spaces  which  once  inclosed  only  fragments  of  country 
rock,  with  numerous  interstices.  These  openings  seem  to  have  been  due  to 
faulting  action  variously  modified  by  local  circumstances.  In  the  Consoli- 
dated Virginia  and  neighboring  mines  a  projecting  mass  upon  the  foot  wall 
gave  rise  to  a  local  rent  in  the  diabase.  In  the  Virginia  group  an  irregu- 
larity in  the  dip  of  the  foot  wall  prevented  the  broken  edge  of  east  country, 
the  great  horse,  from  following  the  main  body  to  its  final  position;  and  a 
crescent-shaped  opening  resulted  which  furnished  an  oj)portunity  for  the 
deposition  of  an  extensive  system  of  bonanzas.     In  Gold  Hill,  on  the  other 


396  GEOLOGY  OF  THE  COMSTOCK  LODE. 

hand,  the  opening  appears  to  be  a  result  of  non-conformity  of  the  wall 
surfaces  brought  into  ojDposition. 

Lateral  secretion. — The  course  of  the  asccnding  waters  appears  to  have 
been  much  influenced  by  the  narrowness  of  the  vein  in  the  lower  levels.  It 
is  highly  probable  that  on  some  straight  or  sinuous  line,  at  depths  greatly 
exceeding  those  yet  reached,  the  vein  is  closed  nearly  water-tight  from  one 
end  to  the  other.  If  so,  water  ascending  in  vast  quantities,  as  it  must  once 
have  done,  would  be  forced  into  the  network  of  capillary  fissures  which 
pervades  the  east  country.  Having  become  saturated  with  soluble  sub- 
stances by  contact  with  the  immense  surface  here  exposed  to  its  action, 
it  would  seek  the  main  fissure  once  more  as  the  path  of  least  resistance, 
and  there  deposit  quartz  and  ore  through  changes  in  physical  conditions,  or 
in  virtue  of  chemical  reactions.  It  is  not  unlikely  that  concentration  by 
evaporation  was  an  important  influence  in  accelerating  precipitation.  The 
character  of  the  deposited  quartz  evidently  varied  greatly  from  time  to  time, 
but  though  the  causes  were  probably  very  simple  in  their  general  nature, 
the  conditions  under  which  they  acted,  considered  in  detail,  must  have  been 
exceedingly  complicated.  On  the  whole,  the  later  deposits  were  probably 
the  richer,  and  it  is  not  impossible  that  a  part  of  the  rich  pockets  and  string- 
ers was  formed  at  the  expense  of  older  deposits  of  lower  grade. 

The  east  wall  of  the  Lode  is  in  most  places  very  indistinct,  though 
occasionally,  as  at  the  Savage  connection  with  the  Sutro  Tunnel,  nothing 
could  be  clearer.  This  is  due  in  part  to  the  percolation  of  strong  currents 
from  the  east  country  during  the  deposition  of  ore,  and  partly  to  dynamical 
action  on  irregular  deposits  crossing  the  lines  of  motion. 

Probabilities  for  lower  depths. — Thc  CoMSTocK  Is  essentlally  a  deposit  at  the  con- 
tact of  diabase  with  underlying  rocks,  and  so  long  as  the  hanging  wall  shows 
a  heavy  body  of  diabase  the  prospects  for  ore  are  good,  mere  depth  not  being 
likely  to  exert  any  prejudicial  eff"ect  upon  the  ore-bearing  character  of  the 
vein.  In  the  search  for  ores  explorations  should  be  confined  to  a  moderate 
distance  from  the  diabase  contact,  for  no  important  bonanza  except  the 
Justice  body  has  been  found  which  does  not  extend  to  within  a  very  short  dis- 
tance from  this  contact;  nor  are  any  bodies  likely  to  occur  far  from  it  which 
will  pay  the  expense  of  discovery.    The  first  condition  for  the  formation  of  a 


SUMMAEY.  397 

quartz  body  is  an  opening  to  receive  it.  The  group  of  mines  worked  through 
the  Union  shaft  and  the  Jacket,  Crown  Point,  and  Belcher  mines  show  peculiari- 
ties of  structure  which  point  to  the  likelihood  of  such  openings  at  lower  levels. 
Openings  such  as  that  which  contained  the  Consolidated  Virginia  and  Cali- 
fornia bonanza,  however,  give  almost  no  warning  of  their  approach  from 
above,  and  may  at  any  time  be  struck  in  the  intermediate  mines;  but  a 
series  of  bodies  nearly  on  one  level,  such  as  were  found  in  the  secondary 
fissure  (the  "Virginia  vein")  is  not  likely  to  recui-. 


THE  THERMAL  EFFECT  OF  KAOLIISQZATION. 

Kaoiinization  hypothesis. — -The  vlew  that  tho  hcat  of  the  CoMSTOCK  is  due  to 
the  kaoiinization  of  feldspar  is  new  and  ingenious,  but  purely  speculative,  for 
there  is  no  unquestionable,  direct  evidence  in  support  of  it;  while  the  process 
is  so  complicated  and  so  little  understood  that  there  is  abundant  room  for 
difference  of  opinion  in  any  discussion  of  the  theory  involved.  Dr.  Barus 
contrived  and  executed  expei^ments  to  test  the  assertion  that  a  rise  of  tem- 
perature followed  the  action  of  heated  waters  from  the  east-country  rock 
of  the  CoMSTOCK.  These  experiments  he  has  described  and  discussed  in 
Chapter  IX. 

The  thermal  effect  of  kaoiinization  may  be  defined  as  the  quantity  of 
heat  generated  by  the  action  of  the  aqueous  vapor  on  the  unit  mass  of  the 
given  feldspathic  rock  in  the  unit  of  time.  It  is  to  be  regarded  as  a  function 
of  the  percentage  quantity  of  feldspar  originally  contained  in  the  given 
rock,  and  of  the  temperature  of  this  material,  as  well  as  of  the  time  during 
which  the  action  has  been  going  on.  A  priori  the  thermal  effect  may  be 
either  positive,  negative,  or  zero.  The  experiments  were  undertaken  to 
ascertain  in  liowfar  the  fundamental  principle  of  the  kaoiinization  hypothesis, 
namely,  that  the  thermal  effect  is  jjositive,  agreed  with  facts.  Such  a  research 
was  also  desirable  because  of  the  intrinsic  interest  which  attaches  to  the 
question. 

Considered  from  a  physical  point  of  view,  the  question  is  rather  a  diffi- 
cult one,  and  of  a  kind  in  which  satisfactory  results  can  be  reached  only 


398  GEOLOGY  OF  THE  COMSTOCK  LODE. 

by  a  laborious  process  of  gradual  approximation.  As  even  in  final  experi- 
ments the  thermal  effect  may  escape  detection,  the  purpose  of  the  first  experi- 
ments may  be  said  to  consist  in  reducing  the  positive  and  negative  limits 
within  which  this  effect  must  lie  to  the  smallest  possible  interval. 

Character  of  the  experiments. — In  proccsses  such  as  kaolinizatiou,  action  may 
usually  be  accelerated  by  an  increase  of  temperature,  provided  that  the  latter 
is  not  sufficient  to  render  the  products  unstable  in  a  normal  case.  In  the 
experiments  it  would  have  been  desirable  to  act  upon  the  rock  with  steam  at  a 
temperature  from  the  boiling  point  of  water  upward,  but  with  the  primitive 
facilities  available  in  Nevada,  the  use  of  superheated  steam  was  not  practi- 
cable. 

The  apparatus  in  which  the  rock  was  subjected  to  thp  action  of  steam 
closely  resembles  that  usually  employed  for  the  determination  of  the  boiling 
point  of  thermometers.  The  rock  to  be  acted  upon  was  crushed  fine  and 
packed  into  a  cyHndrical  receptacle  open  at  the  top,  and  provided  with  a 
wire-gauze  bottom.  This  was  supported  in  the  steam  space  of  a  boiler 
provided  with  an  external  packing.  Th*^  object  of  the  arrangement  was  to 
allow  the  heat,  possibly  generated  in  the  mass  of  rock  by  the  process  of  kao- 
linization,  to  accumulate. 

Measurements  of  temperature. — The  difference  of  tempcraturc  between  the  in- 
terior of  the  rock  and  the  steam  surrounding  it  was  determined  by  the 
aid  of  a  thermopile  consisting  of  three  bismuth-platinum  couples,  one  junc- 
tion being  placed  at  the  center  of  the  pulverized  rock,  and  the  other  in  the 
steam-jacket  surrounding  the  rock  receptacle.  The  electromotive  force  was 
measured  by  a  method  of  compensation.  The  constants  of  the  apparatus 
were  frequently  rechecked,  and  divers  precautions  were  observed  in  the 
experimentation,  and  in  the  mathematical  treatment  of  the  measurements, 
as  is  explained  in  Chapter  IX.  The  means  employed  enabled  the  observer  to 
detect  a  variation  in  the  difference  of  temperature  between  the  two  ends  of 
the  thermopile  as  small  as  0.001°  C. 

Details  of  apparatus  and  method. — The  boilcr  was  heated  by  two  kerosene  stoves, 
each  containing  two  broad  wicks.  The  oil  could  be  replenished  without 
interfering  to  an  appreciable  extent  with  the  flames.  The  water  lost  by 
evaporation  was  replaced  drop  by  drop  by  means  of  a  simple  device,  and  a 


SUMMARY.  399 

glass  water-gauge  indicated  the  progress  of  evaporation.  The  whole  aim 
was  to  make  the  process  a  continuous  one,  and,  had  it  not  been  for  acci- 
dents, a  nearly  constant  source  of  heat  and  a  nearly  constant  water-level 
would  have  made  it  possible  to  keep  up  an  ebullition  of  nearly  constant 
intensity  for  an  indefinite  period  of  time. 

The  rock  used  was  earlier  diabase  from  the  hanging  wall  of  the  Lode 
collected  in  the  main  Sutro  Tunnel.  It  had  undergone  only  a  trifling  amount 
of  decomposition. 

The  experiments  were  continued  during  a  period  of  nearly  five  weeks, 
unfortunately  with  an  accident  between  the  first  and  second,  and  another 
between  the  second  and  third.  On  the  average,  three  observations  of  tlie 
difi'erence  of  temperature  of  the  ends  of  the  thermopile,  or,  say,  T—t,  were 
made  during  each  twenty -four  hours. 

Mathematical  treatment  and  results. — lu  ordcr  to  obtaiu  a  comprchensivc  view  of 
the  large  number  of  data  obtained  it  will  be  sufficient  to  assume  the  empirical 
relation, 

where  a  and  /?  are  constants  to  be  calculated  by  the  method  of  least  squares, 
X  the  time  in  hours  corresponding  to  any  particular  T — t,  and  dated  from 
the  commencement  of  the  series  of  experiments  to  which  the  results  belong. 
Under  variation  of  oc,  an  apparent  thermal  eff'ect  not  due  to  kaolinization 
may  be  conveniently  understood. 

For  a  a  mean  value  of  — 0.05°  C.  was  found.  The  interior  of  the  rock 
was,  therefore,  invariably  colder  than  the  surrounding  steam  It  follows, 
also,  that  it  is  impossible,  even  after  the  lapse  of  a  great  interval  of  time,  to 
heat  so  large  a  mass  of  material  to  an  equal  temperature  throughout.  The 
variation  of  oc  will  add  itself  algebraically  to  /?;  and  unless  the  thermal 
eff'ect  of  kaolinization  is  comparatively  large,  will  entirely  vitiate  the  sig- 
nificance of  the  latter  constant.  /?  gives  nominally  the  rate  of  increase  of 
the  temperature  of  the  interior  of  the  rock  per  hour  in  consequence  of  a 
thermal  effect.  Instead  of  reporting  /?,  however,  it  is  more  expedient  to  give 
the  corresponding  rate  B  referred  to  a  year  as  the  unit,  viz.: 

B  —  S,lQb/3 

For  reasons  which  appear  in  Chapter  IX.  the  experimental  data  may 


400  GEOLOGY  OF  THE  COMSTOCK  LODE. 

be  conveniently  divided  into  two  portions.     In  the  first  of  these  it  was 

found  that 

5=+l°.5d=0°.l; 
in  the  second 

5=-0°.9±0°.l. 

Hence  it  appears  that  the  variation  of  a  alone  was  observed.  The 
vakies  of  B  are  to  be  regarded  as  an  index  of  the  errors  incident  to  the 
method  in  its  present  form,  and  it  is  moreover  probable  that  the  effect  of 
kaolinization  is  negligible  in  comparison. 


THE  ELECTRICAL  ACTIVITY  OF  ORE  BODIES. 

Preliminary  statement. — It  is  Well  known  that  Fox,  Rcich,  and  others  made 
experiments  of  great  interest  upon  the  electrical  phenomena  of  ore  bodies. 
Bernhard  von  Cotta  eai'nestly  recommended  that  these  experiments  should 
be  further  pursued,  as  they  seemed  to  him  likely  to  lead  to  results  of  prac- 
tical importance  in  the  discovery  of  ore  bodies.  If  this  recommendation 
has  ever  been  followed  out,  no  account  of  the  investigation  has  been 
published.  It  was  my  earnest  desire  to  see  the  subject  pursued,  and  Dr. 
Barus  was  invited  to  join  the  Survey  on  account  of  his  special  fitness 
for  this  inquiry.  All  the  plans  and  details  of  the  electrical  surveys  made 
are  due  to  Dr.  Barus,  the  general  scope  of  the  work  and  the  localities  only 
being  prescribed;  and  a  r^sumt?  of  his  results  is  given  below.  Neither  of 
the  localities  chosen  was  the  best  possible  for  the  purpose.  It  was  evi- 
dently necessary  in  such  an  inquir}-  to  begin  by  the  examination  of  ore 
bodies  already  exposed.  At  the  date  of  the  examination*  there  was  very 
little  ore  in  sight  on  the  Comstock.  At  Eureka  large  bodies  of  ore  were 
exposed,  but  being  in  an  oxidized  condition  would  be  likely  to  give  weaker 
currents  than  sulphides  of  similar  quantity  and  distribution.  These  two 
localities,  however,  were  the  only  ones  practically  available,  and  at  the  same 
time  accessible  through  extensive  workings.  The  results  are  nevertheless 
of  great  interest,  and  a  considerable  advance  has  been  made  towards  a  solu- 
tion.   It  is  one  of  the  plans  for  the  future  to  repeat  these  experiments  under 


STJMMAET.  401 

more  favorable  conditions.  The  following  summary  is  in  Dr.  Barus's  own 
words : 

Nature  of  the  problem. — Tlic  problem  offered  is  not  apparently  a  difficult  one, 
and  consists  simply  in  determining  the  variation  of  earth-potential  at  as 
many  points  as  may  be  desirable  within  and  in  the  vicinity  of  the  ore  body ; 
or,  in  other  words,  in  tracing  the  contour  and  position  of  the  equipotential 
surfaces. 

It  is  practicable,  however,  to  systematize  the  method  of  research,  a 
priori.  In  the  first  place,  Reich's  hypothesis  that  lode-currents,  if  present, 
are  due  to  hydro-electric  action  is  quite  a  safe  and  natural  one.  It  is  known 
that  a  number  of  ores — especially  sulphides — possess  metallic  properties. 
The  presence  of  two  or  more  of  these  in  the  same  ore  body  is  not  an 
uncommon  occurrence,  and  electric  action  at  their  surfaces  of  contact 
may  fairly  be  anticipated.  The  currents  thus  generated  have  a  very 
close  analogy  to  those  technically  known  as  "local  currents"  in  batteries, 
which  are  due  to  impurities  in  the  zinc.  In  the  second  place,  it  is  obvious 
that  if  currents  are  met  with  in  a  region  of  ore  deposits,  such  currents  must 
be  constant,  both  in  intensity  and  direction,  because  electrical  action  has 
been  going  on  for  an  indefinite  period  of  time.  The  equipotentials  cor- 
responding to  this  flow  will,  therefore,  have  fixed  and  definable  positions 
relatively  to  the  ore  body. 

Suppose,  now,  that  from  a  point  remote  from  the  ore  body  a  line  has 
been  drawn  towards  it  and  prolonged  beyond  to  about  the  same  distance. 
It  is  not  necessary  for  the  present  purposes  that  this  line  should  actually 
pierce  the  deposit;  but  only  that  certain  of  its  parts  should  be  sufficiently 
near  ore,  and  more  so  than  its  extreme  points,  and  that  it  should  lie  wholly 
within  or  entirely  upon  the  surface  of  the  earth.  Suppose,  moreover,  that 
the  ores  are  so  associated  as  to  generate  electrical  currents. 

If,  then,  beginning  at  one  end  of  the  line  the  values  of  earth-potential 

are  determined  at  consecutive,  approximately  equidistant  points,  it  is  obvious, 

inasmuch  as  the  line  passes  by  the  seat  of  an  electromotive  force,  or,  in 

other  words,  through  the  field  of  sensible  electrical  action,  that  progress  from 

one  extremity  of  the  line  to  the  other  must  be  accompanied  by  a  passage 

of  the  corresponding  values  of  earth-potential,  through  a  maximum  or  min- 
26  c  L 


402  GEOLOGY  OF  THE  COMSTOCK  LODE. 

imum,  or  both,  or  a  number  of  such  characteristic  variations.  In  short,  the 
earth-potential  at  any  point  may  be  regarded  as  a  function  of  the  distance  of 
this  point  from  the  assumed  origin  of  the  hne.  The  assertion  that  this 
function  will  pass  through  a  characteristic  change  of  the  kind  specified  is 
only  another  way  of  stating  that  the  line  may  be  chosen  so  long  that, 
in  comparison  with  its  extent,  the  field  of  sensible  electrical  action  will  be 
local,  or  its  linear  dimensions  in  the  direction  in  question  small.  Maxima 
in  a  general  sense  are,  therefore,  to  be  regarded  as  criteria,  and  as  indicat- 
ing the  part  of  the  line  nearest  to  the  electrically  active  ore  body. 

Practically,  since  we  possess  no  means  of  measuring  potential  abso- 
lutely, it  is  sufficient  to  assume  a  value  (zero)  for  one  of  the  points  of  the 
series.  The  electromotive  force  between  this  and  any  of  the  other  points 
is  then  the  potential  of  the  latter. 

Methods  employed. — lu  making  thc  actual  measurements,  the  simple  problem 
above  enunciated  became  quite  complicated,  because  the  small  lode  electro- 
motive forces  were  afi"ected  by  a  number  of  errors,  which,  in  the  aggregate, 
might  possibly  produce  an  effect  in  the  same  order.  On  the  Comstock, 
where  the  mine  workings  were,  without  exception,  in  very  barren  or  nearly 
exhausted  parts  of  the  vein,  no  definite  evidence  of  currents  due  to  the  Lode 
itself  was  obtained.  Even  at  Eureka,  in  spite  of  the  enormous  ore  bodies  in 
sight,  the  range  of  variation  of  potential  corresponding  to  a  distance  of 
2,000  feet  in  the  underground  experiments  very  rarely  reached  0.1  volt; 
while  usually  this  variation  lay  within  a  few  hundredths  of  a  volt.  These 
limits,  in  a  case  where  such  disturbances  exist  as  action  between  terminals, 
polarization,  earth  currents  (normal),  bad  insulation  of  circuit  at  any  point, 
difference  of  potential  between  liquids  in  contact,  incidental  effects  due  to 
masses  of  metal  distributed  throughout  the  mine,  etc.,  are  to  be  considered 
as  comprehending  a  rather  dangerously  small  interval.  This  small  varia- 
tion of  potential  is  to  be  attributed  to  the  earthy  character  of  the  Eureka 
ores.  For  the  manner  in  which  the  effects  of  the  disturbances  were  to  a 
large  extent  eliminated,  the  reader  must  be  referred  to  Chapter  X. 

Of  the  different  surveys  made,  the  one  on  the  600-foot  level  of  the 
Richmond  mine,  west  drift,  presents  the  greatest  interest,  because  it  was 
here  that  all  the  precautions  necessary  could  be  satisfactorily  applied.     The 


SUMMARY.  403 

line  of  survey,  moreover,  lay  completely  outside  of  the  ore  body,  and  all 
the  points  tapped  were  in  rock  essentially  of  the  same  kind.  The  measure- 
ments were  made  in  various  galvanometric  ways,  and  the  results  were  subse- 
quently checked  by  a  "zero"  method.  It  was  found  that  the  distribution 
of  potential  along  the  length  of  the  drift,  even  after  an  interval  of  four 
months,  had  not  materially  changed,  and  that,  on  passing  from  barren  rock 
towards  and  across  the  ore  body,  small  though  decided  variations  of  poten- 
tial were  encountered  in  its  vicinity. 

Results. — The  electi-ical  effects  observed  were  too  distinctly  pronounced 
to  be  referable  to  an  aggregate  of  incidental  eri-ors,  and  they  were  of  the 
character  which  must  have  been  produced  had  the  ore  bodies  been  the  seat 
of  an  electromotive  force.  The  experiments  made  cannot  be  said  to  have 
settled  the  question  as  to  whether  lode  currents  will  or  will  not  be  of  prac- 
tical assistance  to  the  prospector.  Indeed,  as  yet  it  cannot  even  be  asserted 
with  full  assurance  that  the  currents  obtained  are  due  to  the  ore  bodies. 
What  has  been  observed  is  simply  a  local  electrical  effect  sufficiently  coin- 
cident with  the  ore  body  to  afford  in  itself  fair  grounds  for  the  assumption 
that  these  contained  the  cause.  Giving  the  investigations  of  Fox  and  Reich 
proper  weight,  however,  the  supposition  that  the  currents  in  the  Richmond 
mine  were  not  due  to  the  ore  bodies  is  extremely  improbable.  But,  unfor- 
tunately, they  are  so  weak  as  to  require  an  almost  impracticable  delicacy  in 
the  researches  designed  to  detect  and  estimate  them.  It  is  highly  probable 
that  under  certain  circumstances  more  powerful  currents  are  generated  than 
those  found  at  Eureka.  It  is  not  unlikely,  for  example,  that  galena,  cinna- 
bar, and  the  copper  sulpho-salts  produce  electrical  effects  of  far  greater 
magnitude,  and  that  the  method  might  be  readily  available  for  the  discovery 
of  such  ores.  The  results  thus  give  much  encouragement  to  further  inves- 
tigations in  this  direction. 

Method  proposed. — lu  the  experiments  thus  far  made,  the  variation  of  poten- 
tial along  a  single  Hue  of  electric  survey  only  has  been  determined.  It  is 
obvious,  however,  that  in  order  to  derive  the  full  benefits  from  such  a  method  a 
number  of  these  surveys  must  be  coordinated.  An  endeavor  should  be  made, 
by  passing  toward  and  from  the  ore  body  in  all  directions,  actually  to  deter- 
mine the  contours  and  positions  of  the  equipotential  surfaces.     It  is  not  im- 


404  GEOLOGY  OF  THE  COMSTOCK  LODE. 

probable  that  the  interpretation  of  the  results  would  furnish  clews  for 
the  economical  exploitation  of  mines,  comparable  in  value  to  those  of  a 
purely  geological  character.  Both  should  go  hand  in  hand.  Under  ground, 
this  general  method  of  research  is  not  always  feasible,  as  it  presupposes  that 
the  mine  has  been  already  widely  exploited.  On  the  surface  of  the  earth, 
however,  it  may  to  some  extent  be  applied;  and  in  this  case  the  endeavor 
would  be  to  obtain  the  traces  of  the  equipotential  surfaces  on  that  of  the 
earth.  Suppose,  for  instance,  that  the  potential  at  every  point  in  several 
square  miles  of  the  earth's  surface  were  known.  Then  let  this  surface 
be  projected  on  a  fixed  horizontal  ("Xr")  plane,  and  the  value  of  earth- 
potential  corresponding  to  each  of  the  points  be  constructed  as  "Z."  In 
this  way  a  new  (potential)  surface  would  be  obtained  coextensive  horizon- 
tally with  the  former.  Terrestrial  electrical  action  would  manifest  itself 
upon  the  new  surface  as  a  whole  and  would  not  affect  its  regularity.  Local 
action,  on  the  other  hand,  would  produce  an  effect  circumscribed  in  com- 
parison with  the  horizontal  extent  of  the  area  under  consideration.  We 
should  expect  to  find  a  hillock  or  depression,  or  both,  or  a  number  of  these 
inequalities  in  the  imaginary  potential  sui'face. 


NOTE   TO   CHAPTER  III. 


FELDSPAE  DETBEMINATIONS  BY  SZABO'S  METHOD. 

In  the  present  state  of  lithological  science  it  is  most  desirable  both  to  determine 
the  feldspathic  constituents  of  rocks  with  accuracy  and  to  bring  the  evidence  of  inde- 
pendent methods  to  bear  for  this  purpose.  Where  rocks  are  extremely  coarsegrained, 
and  at  the  same  time  carry  feldspars  the  solidity  of  which  is  unimpaired  by  cracks, 
the  results  of  the  examination  of  cleavage  flakes  under  the  microscope  leaves  little  to 
be  desired  ;  but  such  rocks  are  exceptional.  The  determination  of  feldspars  in  rock- 
slides  is  subject  to  two  disadvantages.  Crystals  of  above  a  millimeter  in  diameter  are 
very  likely  to  be  broken  in  grinding,  and  thus  to  escape  examination ;  and  though  the 
microscopist  may  often  infer  the  presence  of  two  or  more  feldspars,  he  can  be  abso- 
lutely certain  only  of  the  most  basic  species  present. 

Szab6's  method,'  on  the  other  hand,  discriminates  with  great  delicacy,  not  only 
the  well-established  feldspar  species,  but  also  the  mixed  feldspars,  perthite  and  loxo- 
clase,  and  the  intermediate  feldspars,  andesine  and  bytownite,  as  to  the  independence 
of  which  mineralogists  are  not  agreed.  It  is  also  particularly  applicable  to  the  larger 
feldspars,  so  seldom  obtained  in  perfect  form  in  slides.  The  weakness  of  the  method 
Ues  in  the  fact  that  it  is  not  applicable  to  very  fine-grained  rocks,  or  to  the  minute  feld- 
spars of  coarser  rocks,  unless  a  separation  has  first  been  effected  by  Thoulet's  method; 
but  this  limitation  does  not  prevent  its  being  excellently  adapted  to  confirm  and  sup- 
plement the  results  of  microscopic  examination. 

At  the  time  of  writing  Chapter  III.  I  did  not  feel  competent  to  apply  Szabo's 
method,  never  having  had  an  opportunity  of  seeing  it  carried  into  practice ;  but  while 
the  proofs  of  this  volume  were  under  correction.  Professor  Szabo  visited  the  country 
and  was  good  enough  to  illustrate  his  method  experimentally  to  some  of  the  members 
of  the  Survey,  including  myself.  After  convincing  myself  of  the  accuracy  of  the 
results  obtainable  and  acquiring  familiarity  with  the  manipulation  by  repeatedly  test- 
ing a  series  of  classical  feldspars,  such  as  anorthite  from  Monte  Somma,  labradorite 
from  Labrador,  orthoclase  from  Baveno,  etc.,  I  proceeded  to  an  examination  of  the 
feldspars  of  the  Washoe  rocks,  the  results  of  which  are  given  below.  From  five  to 
ten  crystals  in  each  specimen  mentioned  were  tested,  and  no  results  obtained  are 
omitted. 

•  Joseph  Szal)6,  Ueber  eine  neue  Methode  die  Feldspathe  auch  in  Gesteiuen  zu  bestimmen.  Buda- 
Pesth,  Franklin-Verein,  1876.     See,  also,  Fouqu^  et  L€vy,  Min^ralogie  Micrographique,  p.  108. 

(405) 


406  GEOLOGY  OF  THE  OOMSTOCK  LODE. 

Granite,  close  to  Red  Jacket  mine. 

The  orthoclastic  feldspar  gave  reactions  corresponding  to  perthite  or  loxoclase, 
showing  a  mixture  of  amazonite  with  a  triclinic  feldspar.  This  mixture  is  also  readily 
recognizable  under  the  microscope.  The  triclinic  crystals  are  in  part  oligoclase  and 
in  part  answer  to  andesine.'  Under  the  microscope  I  have  noticed  no  crystals  more 
basic  than  oligoclase. 

Granular  diorite,  Bullion  Eavine  at  Water  Company's  flume. 

All  the  feldspars  tested  gave  reactions  for  andesine,  with  a  tendency  rather  towards 
labradorite  than  towards  oligoclase.  Under  the  microscope  the  maximum  angles  found 
answered  to  labradorite,  but  the  occurrence  of  zonal  structure  was  noted.** 

Granular  diorite,  Utah,  1950. 

In  this  specimen  labradorite,  andesine,  and  crystals  of  intermediate  composition 
were  found. 

Porphyritic  diorite,  Ophir  Ravine,  south  side. 

Labradorite  and  andesine  only  were  detected. 

Metamorphic  diorite,  Amazon  dump. 

All  the  feldspars  tested  were  oligoclase,  according  with  the  microscopic  results. 

Quartz-porphyry,  1,000  feet  south  of  Lawson's  Tunnel. 

Amazonite  and  oligoclase  were  found,  as  well  as  a  feldspar  slightly  more  basic 
than  oligoclase,  but  not  so  much  so  as  andesine.  This  mineral  is  therefore  much  more 
closely  allied  to  oligoclase  than  to  labradorite.  No  angles  of  extinction  exceeding 
those  of  oligoclase  were  observed  in  the  slide.  Oligoclase  was  also  found  in  the  rock 
described  on  page  109  (slide  304). 

Earlier  diabase,  Sutro  Tunnel,  north  branch,  50  feet  south  of  Ophir. 

All  but  one  of  the  crystals  tested  proved  to  be  labradorite.  The  exception  was 
an  andesine. 

Earlier  horublende-andesite,  North  Twin  Peak. 

A  single  feldspar  had  a  composition  intermediate  between  labradorite  and  ande- 
sine. The  remainder  were  characteristic  labradorites.  The  zonal  feldspar  described 
on  page  61,  and  shown  in  Plate  III.,  Fig.  13,  is  from  the  same  cropping,  though  not 
from  the  same  specimen. 

Earlier  hornblende-andesite,  1,200  feet  northwest  of  Geiger  Grade  Toll  House. 

The  microscopic  examination  led  to  the  sui^position  that  anorthite,  labradorite, 
and  oligoclase  were  all  present,  the  last,  however,  only  as  microlites.  The  crystals 
tested  by  Szabo's  method  proved  to  be  andesine  and  a  feldspar  intermediate  between 
this  and  labradorite.    No  anorthite  was  met  with.    This  fact,  however,  of  course  does 


'  It  is  usual  to  regard  andesine  as  peculiar  to  volcanic  rooks,  and  the  plagioclase  of  granite  is 
often  supposed  to  be  exclusively  oligoclase.  Professor  Rosenbusch,  however  (Physiog.  der  Massigeu 
Gest.,  II.,  121),  mentions  finding  plagioclaaes  in  granites  which  showed  angles  of  extinction  correspond- 
ing to  all  of  the  feldspar  species  excepting  anorthite. 

'  For  some  remarks  on  the  indications  of  zonal  structure,  see  page  61. 


FELDSPAR  DETEEMINATIONS.  407 

not  ijrove  that  none  is  present  in  the  rock,  but  only  that  it  is  comparatively  infrequent. 
That  it  was  not  the  predominant  feldspar  was  also  inferred  from  the  microscopic 
examination. 

Augite-andesite,  peak  south  of  Crown  Point  Ravine,  marked  7075. 

Anorthite,  bytownite,  and  labradorite  were  detected  in  this  specimen.  The 
anorthite  was  found  under  the  microscope,  and  its  presence  prevented  the  detection  of 
labradorite,  except  among  the  microlites.  But  on  page  64  it  is  stated  of  the  augite- 
andesite  that,  though  "  anorthite  has  been  identified  in  a  few  slides,  •  *  *  in  most 
cases  the  maximum  angles  of  extinction  correspond  to  labradorite." 

Augite-andesite,  above  Ophir  grade,  due  west  of  Belcher  hoisting- works. 

This,  too,  showed  anorthite,  labradorite,  and  an  intermediate  variety.  Anorthite 
was  found  also  in  the  augite-andesite  from  Basalt  Hill. 

Later  hornblende-andesite,  quarry  2,000  feet  northeast  of  Sutro  Tunnel  Shaft  III. 

All  of  the  feldspars  tested  (more  than  a  dozen)  gave  very  sharp  reactions  for 
andesine.^    Nearly  all  of  them  show  zonal  structure. 

Later  hornblende-andesite,  quarry  2,000  feet  east  of  Occidental  Mill. 

An  andesine,  an  oligoclase,  and  several  crystals  of  intermediate  composition 
were  found.  This  accords  excellently  with  the  analyses  of  these  feldspars  made  by 
Mr.  Dewey.* 

Later  hornblende-andesite,  quarry  above  Utah  mine. 

Labradorite,  andesine,  and  an  intermediate  variety  were  detected. 

It  was  not  found  practicable  to  examine  the  feldspars  of  the  later  diabase  (black 
dike)  or  the  basalt,  on  account  of  the  fine  grain  of  these  rocks. 

On  the  whole,  the  examination  strongly  confirms  the  results  of  the  microscopical 
analysis,  and  the  only  rocks  in  which  the  flame-reactions  revealed  feldspars  which 
might  have  been  detected  by  the  microscopic  method  are  the  granite  and  the  quartz- 
porphyry,  each  of  which  shows,  in  addition  to  oligoclase,  an  unsuspected,  more  basic 
feldspar,  which  is,  however,  more  closely  allied  to  oligoclase  than  to  labradorite. 

While  the  optical  behavior  of  a  few  of  the  large  feldspars  in  the  Washoe 
andesites  indicates  that  they  are  mixtures  of  different  species,  this  is  exceptional. 
They  are  ordinarily  polysynthetic  individuals,  showing  only  two  angles  of  extinction. 
In  a  large  proportion  of  cases  the  crystallographic  relations  of  the  twinned  lamellae 
are  further  emphasized  by  the  presence  of  zonal  structure.  Granting  the  accuracy  of 
Szabo's  method,  it  is  therefore  extremely  difficult  to  suppose  that  the  prevalence  of 
crystals  giving  the  flame-reactions  for  andesine  in  the  andesites,  and  particularly  in 
the  rock  from  the  quarry  northeast  of  Sutro  Shaft  III.,  is  due  to  aggregations  of  two 
distinct  species.    The  uniformity  of  the  reactions  obtained  is  also  an  argument  against 

1  Professor  Szabd  examined  two  crystals  from  this  specimen,  and  prononnoed  them  very  charac- 
teristic andesine. 

"  See  pages  67  and  154. 


408  GEOLOGY  OF  THE  COMSTOCK  LODE. 

such  a  supposition.  In  examining  some  fine  instances  of  so-called  perthite  from  the 
original  Canadian  locality  and  from  others  on  the  Atlantic  coast  I  have  found  the 
reactions  extremely  variable,  depending,  as  one  cannot  but  suppose,  on  the  relative 
proportions  of  the  two  component  feldspars  which  happened  to  be  present  in  the  little 
fragment  tested.  In  the  andesite  referred  to,  on  the  other  hand,  the  reactions  of  the 
feldspars  were  as  regular  as  those  obtained  from  different  fragments  of  a  single  stand- 
ard feldspar.  The  facts,  therefore,  do  not  appear  to  me  to  warrant  the  supi^osition 
either  that  these  crystals  are  mixtures  of  different  feldspar  species  independently 
crystallized  or  that  they  correspond  in  composition  to  some  one  of  the  unquestioned 
varieties.  They  must  rather  be  set  down  as  isomorphous  mixtures,  in  the  sense  in 
which  that  term  is  to  be  understood  in  Tschermak's  theory  of  feldspar-composition. 


INDEX    TO    MTNINa    CLAIMS. 

[Atlas-sheet  rrr. — Map  of  the  Wabuob  District  showing  Mining  Claims.] 


Mine. 


Latitude, 
N.39°. 


Minutes.  Seconds. 


Alabama 

Alexander 

Alhambra 

AUen,  or  Peruvian. . 

Alpha 

Alpine 

Alta 

Alta  (Patent) 

Amazon 

America 

American — 

American 

American  Eagle 

American  Flat 

Andes 

Andrews 

Arctic 

Argonaut 

Arizona 

Atlantic 

Bailey 

Baltic 

Baltimore  American 
BaltimoreConsolidated 

Belchapin 

Belcher 

Belcher  Extension  ... 

Benjamin 

Benton 

Best  &  Belcher 

Bluejacket. 

Boehler 

Bonanza  

Boyle 

Brilliant 

Browne 

Buckeye 

Bullion 

Caldwell 

Caledonia 


18 
17 
16 
15 
19 
17 
17 
16 
16 
14 
19 
14 
15 
14 
16 
18 
18 
19 
15 
16 
15 
18 
16 
16 
16 
16 
17 
16 
15 
16 
18 
19 
14 
18 
16 
16 
19 
16 
17 
17 
18 


Minutes.  Seconds 


53 
50 
27 
05 
27 
40 
SO 
30 
35 
45 
07 
45 
15 
45 
30 
40 
25 
50 
50 
50 
40 
00 
35 
40 
35 
45 
15 
45 
45 
35 
27 
25 
05 
55 
42 
50 
20 
00 
50 
15 
50 


Longitude, 
W.  119°. 


37 
38 
38 
38 
39 
38 


37 
41 
38 
38 
40 
39 
38 
37 
37 
40 
38 
37 
40 
40 
40 
39 
39 
39 
39 
39 
38 
38 
38 
38 
38 
38 


15 
25 
46 
16 
30 
15 
20 
00 
55 
40 
20 
00 
25 
60 
20 
00 
13 
20 
43 
30 
55 
25 
38 
10 
05 
15 
30 
18 
16 
00 
50 
22 
30 
58 
50 
57 
17 
35 
08 
39 
60 


Mine. 


Minutes.  Seconds. 


California  

California 

California  Bank 

Capital 

Capital  No.  2 

Carolina 

Carolina 

Carson 

Cavonr  

Cherokee 

Chollar 

Chonta 

Chonta 

City  of  Melbourne 

Clemens 

Cliff  House 

Clyde 

Cole 

Colorado  

Colossus 

Columbia 

Columbia 

Columbia 

Comet 

Comet  Extension  .... 

Compromise 

Concordia   

Confidence 

Consolidated  Virginia 

Cook  &  Gray 

Cosmopolitan 

Cosmopolitan 

Coso 

Coupon 

Coupon  No.  2 

Coye 

Crevice ". 

Cromer  .  

Crown  Point 

Crown  Point  Extension 
Crown  Point  Ravine 


Latitude, 
N.  39°. 


18 
16 
19 
16 
16 
19 
16 
15 
19 
15 
18 
16 
16 
17 
IB 
16 
16 
18 
19 
19 
19 
14 
14 
IS 
15 
16 
18 
17 
18 
16 
18 
16 
16 
IS 
15 
14 
17 
15 
17 
17 
17 


Minutes. 


40 
20 
40 
30 
50 
35 
18 
00 
27 
30 
05 
45 
28 
20 
52 
00 
50 
50 
20 
40 
30 
10 
05 
30 
15 
40 
35 
35 
30 
10 
10 
30 
20 
15 
05 
30 
20 
66 
20 
05 
30 


Longitude, 
W.  119°. 


38 
39 
38 
38 
38 
37 
38 
37 
37 


39 

39 


38 
39 
39 
38 
37 
37 
38 
38 


38 


38 


38 
39 
39 
39 


Seconds. 


60 
30 
00 
43 
45 
62 
08 
60 
10 
17 
00 
12 
07 
20 
12 
30 
00 
18 
00 
00 
25 
22 
26 
61 
55 
20 
26 
20 
50 
21 
15 
00 
40 
55 
66 
25 
06 
10 
20 
18 
33 


(409) 


410 


GEOLOGY  OF  THE  COMSTOCK  LODE. 


Index  to  mining  claims — Continued. 


TWinft. 


Latitude, 
N.39°. 

Longitude, 
W.  119°. 

Minutes. 

Seconds. 

Minutes. 

Seconds. 

19 

20 

37 

05 

17 

20 

38 

30 

17 

15 

38 

37 

17 

10 

39 

10 

14 

40 

38 

10 

19 

45 

36 

55 

16 

55 

39 

27 

16 

15 

38 

02 

15 

20 

38 

11 

14 

20 

38 

15 

16 

50 

37 

40 

15 

15 

39 

OS 

IS 

55 

41 

15 

15 

30 

38 

40 

18 

50 

37 

30 

18 

10 

37 

35 

19 

27 

37 

46 

14 

45 

38 

15 

15 

20 

38 

20 

16 

50 

39 

15 

16 

55 

39 

15 

14 

55 

38 

40 

17 

35 

38 

37 

15 

30 

39 

16 

19 

35 

37 

30 

15 

50 

38 

28 

16 

10 

38 

15 

16 

15 

39 

05 

16 

40 

39 

30 

17 

10 

38 

25 

18 

15 

38 

10 

19 

50 

36 

45 

15 

36 

38 

10 

18 

65 

37 

15 

15 

06 

38 

35 

15 

30 

38 

30 

17 

30 

38 

45 

17 

46 

39 

15 

17 

25 

38 

30 

18 

45 

38 

15 

16 

20 

38 

10 

15 

20 

40 

30 

19 

40 

37 

10 

16 

33 

40 

20 

16 

15 

37 

30 

14 

55 

38 

20 

16 

55 

39 

10 

19 

35 

37 

55 

16 

55 

39 

06 

14 

55 

38 

10 

15 

55 

40 

40 

Mine. 


Minutes.  Seconds. 


Latitude, 
N.390. 


Minutes.  Seconds. 


Longitude, 

"W.  1190. 


Crystal  Eidge  

Cuiver 

Culver  Addition 

Curtis 

Daney  

Daniel  Webster 

Dardanelles 

Dardanelles 

Dayton 

DaytonNo.2  

Dean 

De  Forest 

Delaware 

DelEey 

Dexter 

Dexter 

Diamond 

Dies  Senor 

Drexel 

E.  Cliapin 

E.  Uomstock 

Edinburgll 

E.  Europa 

Elevator 

ElUot 

E  migrant 

Enderwood 

English  Company 

Enterprise 

E.  Overman 

E. Savage 

Esperance  

Esper.iuza    

Essex 

E.Star 

Eslelle 

Europa 

Excliequer. 

E.  Yellow  Jacket 

Fairfax 

Flora  Temple 

Florida 

Francisco  Mareano  . . 

Frankel 

Frauklin-German 

French 

Fi-ont   Lode  Consoli- 
dated   

Fry 

Fry 

Genesee 

Georgia 


German 

German 

Gila  Mina 

Glasgow 

Glen 

Globe 

Golden  Arrow 

Gold  Hill  Company . . 
Gold  Hm  Tunnel  . . . . 

Gold  Lead 

Gold  Leaf 

Goodman 

Gould  &  Curry 

Grant 

Great  Eastern 

Great  Western 

Green 

Grosh 

Grosh  Consolidated . . 

Hale  &  Norcross 

Hale  &  Horcross,  S.  E. 

Extension 

Hardy 

Harlem    

Hartford 

Hawkeye 

Hawley  Consolidated 

Hayward 

Hector 

Henry  Clay 

Hercules 

Hillside 

Holman 

Imperial 

Independent 

Industry 

Insurance 

Iowa 

Irving 

Jackson  

Jacob  Little    

James 

Joe  Scates 

Joe  Scates  

John  Auer 

Julia 

Julia,  E.  Extension. . . 

JuliaNo.2 

Jura 

Justice 

Kate 

Kennebec 


16 
16 
16 
14 
19 
16 
19 
16 
17 
19 
15 
15 
18 
IS 
18 
14 
15 
17 
17 
18 

17 
18 
15 
16 
16 
14 
17 
19 
19 
18 
15 
16 
17 
19 
15 
18 
19 
19 
15 
19 
19 
18 
18 
18 
17 
17 
17 
16 
16 
18 
18 


20 
15 
05 
50 
40 
40 
10 
40 
30 
10 
35 
40 
20 
50 
05 
35 
50 
35 
33 
10 

45 
45 
30 
05 
20 
05 
27 
15 
37 
35 
15 
20 
38 
55 
35 
45 
00 
45 
10 
10 
50 
30 
15 
18 
60 
60 
50 
55 
20 
35 
SO 


38 
38 
38 
38 
37 
40 
37 
37 


39 
38 
38 
37 


39 


39 
38 
37 


38 
37 
37 
38 
38 
39 
38 
37 
39 
38 
38 
38 
38 
38 
38 
38 
37 
38 
38 
38 
40 
39 
37 
37 


18 
20 
32 
30 
20 
38 
10 
25 
10 
46 
35 
10 
SO 
10 
30 
15 
40 
00 
50 
00 

40 
35 
10 
55 
30 
15 
05 
15 
00 
35 
35 
30 
20 
15 
50 
02 
50 
37 
20 
48 
20 
26 
33 
30 
50 
25 
35 
20 
02 
35 
4S 


CLAIM  MAP. 


411 


Index  to  mining  claims — Continued. 


Mine. 


Latitude, 
N. 39°. 


Minutes.  Seconds. 


Keystone 

Keystone 

Knickerbocki-i-    

Kossuth 

Kossuth  Extension  . . 
Lady  Wasliingtoii    . . 

La  Fayette 

Lanzac  

La  Plata 

Lassen 

Lawson 

Leo 

Leviathan 

Lexington 

Lincoln 

Little  G-iant 

Little  York 

Lookout 

Lord  of  Lome 

Lowery 

Low  Kange 

Lucorue 

Mackey 

Manhattan  Consoli- 
dated   

Margarita 

MargaritaNo.2 

Marsano 

Marvel 

Mary 

Mary  Ar^n 

Maryland 

McErlain 

McGinnis  &  Bazaii  .  - 

McKibben 

Memuon 

Memphis 

Metela 

Metropolitan 

Mexican 

Miami 

Midas 

Mill  Site 

Mill  Site 

Mill  Site 

MiUSite 

Mint 

Mississippi 

Miasouii 

Mitchell 

Modoc  Chief 

Monte  Christo  No.  2  . 


20 
16 
16 
15 
15 
16 
18 
15 
17 
19 
16 
16 
17 
16 
19 
16 
19 
11 
15 
16 
18 
16 
19 

19 
19 
19 
19 
19 
18 
18 
16 
16 
16 
18 
14 
16 
20 
15 
18 
18 
16 
16 
16 
16 
16 
18 
16 
17 
16 
17 
18 


Minutes.  Seconds. 


15 
35 
48 
00 
15 
30 
25 
30 
15 
40 
30 
05 
10 
00 
20 
03 
25 
45 
00 
55 
20 
07 
10 

55 
20 
15 
35 
10 
15 
50 
40 
10 
20 
42 
10 
35 
02 
00 
50 
40 
00 
49 
45 
35 
20 
10 
OS 
35 
17 
20 
20 


Longitude, 
W.  119°. 


38 
39 
40 
38 
38 
39 
37 
38 
39 
37 
40 
38 
39 
39 


38 
37 
39 
38 
38 
38 
37 

30 
37 
37 
37 
38 
38 
38 
40 
38 
37 
39 
38 
39 
38 
38 
38 
38 
38 
38 
40 
37 
40 
38 
38 
38 
38 


10 
10 
05 
05 
00 
05 
30 
45 
40 
35 
40 
35 
20 
07 
65 
30 
45 
40 
40 
00 
24 
50 
05 

30 
25 
20 
15 
10 
15 
05 
00 
10 
40 
12- 
15 
03 
10 
10 
45 
10 
20 
25 
25 
37 
20 
30 
25 
25 
47 
45 
10 


Mine. 


Latitude, 
N.  39°. 


Minutes.  Seconds. 


Monte  Cristo 

Montezuma 

Monumental 

Mooney  &  "Whiteman. 

Moore  '&  Morga  n 

Morning  Star  No.  2    . . 

Mountain  View 

Nagle 

N.  Chipman 

N.  Comstock 

N.  Consolidated  Vir- 
ginia  

N.  Dayton 

Nevada 

Nevada  No.  3 

New  Empire  State 

New  Oregon 

New  York 

New  York  Mill  Site  . . 

Niagara 

Nigger  Ravine 

N.  Knickerbocker 

N.  Lexington 

N.  Mexican 

N.  Milton 

N.  Occidental 

N.  Ophir 

North 

Northern  Light 

Northern  Light 

N.  Prospect 

N.  Star 

N  Star 

Occidental 

Ohio 

Ontario 

Ophir 

Original  Gold  HUl .... 

Orleans 

Oro 

Oro 

Overman 

Overman  No.  2 

Overton 

Palmetto 

Patten 

Pearson 

Peruvian,  or  Allen 

Peytona 

Phcenix-Westem 

Pictou 

Pioneer 


18 
18 
17 
14 
17 
18 
19 
16 
17 
19 

18 
15 
16 
16 
20 
16 
16 
16 
16 
16 
17 
16 
19 
19 
17 
19 
20 
18 
18 
17 
19 
15 
17 
19 
17 
18 
17 
19 
19 
17 
17 
16 
19 
18 
16 
16 
19 
19 
15 
16 
10 


Minutes.  Seconds. 


35 
30 
10 
25 
35 
57 
15 
00 
17 
25 

55 
20 
30 
25 
30 
50 
45 
43 
05 
00 
00 
20 
08 
20 
20 
05 
15 
20 
53 
20 
15 
40 
05 
50 
20 
45 
35 
50 
65 
12 
05 
40 
00 
42 
35 
25 
27 
25 
16 
10 
20 


Longitude, 
W.  119°. 


37 
37 
38 
38 
38 
37 
37 
40 
38 
38 

38 
38 
40 
40 
38 
40 
39 
39 
38 


39 
38 
37 
37 
38 
38 
37 
38 
37 
37 
40 
37 
37 
39 
37 
39 


39 
39 
39 
39 
39 
40 


38 
37 
39 
37 


15 
40 
50 
15 
30 
30 
45 
10 
47 
10 

05 
30 
30 
24 
15 
05 
07 
02 
40 
00 
50 
10 
05 
40 
43 
15 
50 
20 
00 
55 
15 
40 
43 
40 
50 
50 
28 
30 
45 
40 
35 
25 
20 
12 
00 
SO 
30 
35 
50 
20 
50 


412 


GEOLOGY  OF  THE  COMSTOC^K  LODE. 


Index  to  mining  claims — Continued. 


Mine. 


Minutee.  Seconds. 


Piute « 

Plato 

PlutuB 

Porphyry 

Potosi 

Pnde  of  Washoe 

Prospect 

Red  &  White  Cross. . 

Keno 

Rock  Island 

Rocky  Bar 

Roman  Capital 

R.  R.  Consolidated  . . . 

Sacramento 

Sadie 

SallieHart 

San  Francisco 

Santiago 

Savage 

S.  Belcher 

S.  California 

S.  Chipman 

S.  Consolidated  Vir- 
ginia  

Scorpion 

Sec.  Line 

Seg.  Belcher 

Senator 

Sewell  &Sheel 

S.Grosh 

Shamo 

Shanley 

Sheridan 

Sherwood 

Sierra 

Sierra  Nevada 

Silverado 

Silver  Central 

Silver  Hill 

Silver  Leaf 

Silver  Leaf 

Silver  Star 

Solid  Silver 

South  Buckeye 

South  Comstock 

South  End 

South  Jacket 

South  Lucerne 

South  Overman 

South  St.  Lonis 

Southern  Star 

Stevens 

St.  John's 


Latitude, 
N.  390. 


Minntes.  Seconds. 


16 

40 

16 

00 

18 

35 

20 

10 

17 

55 

18 

20 

17 

10 

18 

20 

18 

35 

16 

15 

18 

40 

18 

25 

17 

50 

19 

10 

18 

30 

19 

30 

15 

65 

15 

05 

18 

15 

17 

00 

16 

60 

17 

10 

15 
19 
16 
17 
18 
14 
17 
18 
18 
19 
16 
16 
19 
16 
15 
16 
17 
15 
16 
19 
15 
15 
15 
17 
15 
16 
15 
15 
16 
17 


40 
05 
48 
10 
00 
40 
20 
45 
38 
15 
50 
OS 
10 
05 
00 
10 
30 
45 
55 
05 
10 
55 
30 
00 
55 
30 
40 
15 
26 
30 


Longitude, 

w.  1190. 


38 
38 


40 
38 
38 
38 
38 
37 
38 
40 


39 
38 


39 
37 
39 


37 
39 
38 
37 
38 


38 
37 
37 
38 
38 
38 
37 
38 
37 
38 
39 
38 
38 
39 


48 
05 
20 
45 
05 
05 
00~ 
20 
10 
37 
00 
28 
20 
43 
10 
10 
25 
20 
55 
18 
15 
46 

05 
50 
25 
30 
35 
47 
00 
13 
45 
25 
53 
53 
20 
45 
56 
50 
15 
30 
20 
55 
35 
30 
00 
35 
45 
25 
45 
40 
55 
35 


Mine. 


Minutes.  Seconds. 


St.  Lawrence 

St.  Loiiis 

St.  LoniB 

Storey  

Storey  

Succor 

Sullivan 

Sunrise 

Sonrifle 

Superior 

Sutro 

Swan 

Table  Mountain 

TamO'Shanter 

T.&C.  Brooks 

Tarto 

Tehama 

Thomberg 

Thornton 

Troy  Consolidated  . . . 

Tucker 

Twin 

Twin  Peaks 

Tyro 

Union  Consolidated . . 

Utah 

Utah 

Utah,  1st  N.  E.  Exten- 
sion  

Utah,  2d  N.  E.  Exten- 
sion  

Venis 

Vermont 

Victoria  Garber 

Virginia  Standard  . . . 

Vivian 

Volcano 

Vulcan 

■Ward 

Ward 

Wasatch 

Washoe  Consolidated 

Waters 

W.  Belcher 

W.  Crown  Point 

Webber    .. 

Wells-Pargo 

Western 

W.  Justice 

Woodville 

W.Star 

Yankee 

Yellow  Jacket   


Latitude, 
N.390. 


14 
19 
16 
17 
17 
16 
16 
19 
17 
17 
19 
16 
15 
15 
14 
16 
17 
19 
19 
19 
17 
16 
17 
20 
18 
19 
16 


19 
18 
18 
19 
18 
16 
15 
15 
19 
17 
18 
16 
19 
17 
17 
14 
19 
15 
16 
16 
15 
16 
17 


35 

35 
00 
05 
00 
10 
50 
05 
00 
35 
30 
40 
35 
50 
10 
13 
30 
50 
00 
35 
00 
00 
05 
15 
55 
35 
55 

40 

50 
55 
48 
05 
40 
15 
20 
30 
45 
38 
25 
35 
32 
20 
15 
40 
30 
"30 
20 
20 
06 
60 

ao 


I^meitude, 
W.  1190. 


Minutes.  Seconds. 


37 
38 
37 
39 
38 
38 
37 
37 
39 
38 
40 


38 
38 
37 
37 
38 
38 
39 


38 
40 

38 

38 
38 
38 
37 
37 
38 
39. 
38 
*38 
38 
38 
39 
37 
39 
39 
38 
37 
38 
39 
38 


30 


50 
46 
42 
15 
00 
31 
45 
35 
30 
56 
40 
20 
23 
34 
06 
52 
11 
30 
50 
40 
20 
10 
05 
40 
46 
30 
30 


00 
05 
20 
35 
05 
10 
00 
35 
20 
45 
05 
55 
35 
40 
35 
00 
45 
10 
10 
56 
45 
20 
M 


GENERAL    INDEX. 


(To  facilitate  the  use  of  the  geologioal  map,  Atlaa-sheet  IV.,  the  positions  of  the  more  noteworthy  localities  mentioned 

in  the  text  are  indicated  in  the  index  by  letters  and  nnmbcrs  attached  in  parentheses.    The  map  is  provided  with  a  cor- 
responding series  of  letters  and  numbers  answering  to  each  minute  of  surface.] 

Page. 
Bams,  Carl,  computations  by 245 

his  experiments  on  kaolinization 236, 397 

on  the  electrical  activity  of  ore  bodies .  290, 400 

Basalt,  description  of  slides  of 134 

lithological  description  of 70 

occurrence  and  age  of  the 205,373,383 

Basalt  Hill  (B.  G),  augite-andesite  of 71,  201 

metamorphic  diorite,  3, 000  feet  SE.  of 107 

Belcher,  assay  of  slate  from  the 155 

cross-section  through  the 277 

fault  at  the 278 

later  diabase  of  the 33, 152 

ore  on  the  3, 000- foot  level  of  the 278 

quartz-porphyry  of  the 196 

slates  in  the 191 

Belts,  mineral,  west  of  the  Kocky  Mountains 2 

Berkshire  CaSoD,  propylite  from 143 

Bisilicates,  course  of  the  decomposition  of  the. ,  214 

Black  border  of  hornblende.    {<See  illustrations.) 
Black  dike.    {See  Diabase,  later.) 

occurrence  and  age 199 

Blake,  "W.  P.,  on  mineral  belts  west  of  the  Rocky  Mount- 
ains         2 

Boiler  (kaolinization),  description  of 292 

Bonanza,  definition  of 268 

of  the  Consolidated  Virginia  and  OcUifomia 

mines 270 

of  the  Consolidated  Virginia  and  California, 

comb-structure  in  the 270 

of  the  Consolidated  Virginia  and  California, 

origin  of  the  opening  occupied  by  the  274 

of  the   Consolidated  Virginia  and  California, 

rock  fragments  in  the 270 

Bonanzas.    {See  Ore.) 

character  of  openings  occupied  by 395 

occurrence  of,  on  the  Comstock 218 

proportions  of  gold  and  silver  in  yield  from,  by 

groups 9 

Virginia  group  of 274, 275 

Bristol,  E.  S.,  analysis  by 152 

Bullion.     {See  Gold,  Silver,  product.) 

Bullion,  ijuartz  of  the 17 

V.  Kichthofen  on  the  prospects  of  the 23 

Bullion  Kavine  {C.  3),  assaysof  granular  diorite  from  the    154 

diorite  of 39,41,93,150,406 

(413) 


Alteration  of  minerals  in  »i(u,  von  Bichthofen  on 20 

A  malgamation,  barrel 6 

pan   6 

Atnazon  {T).  7),  analysis  of  metamorphic  diorite  from  the, 

tiible  following  page  151 
analysis  of  metamorphic  diorite  from  the.  155 

metamorphic  diorite  of  the 43, 106, 196, 406 

American  Flat  (B.  5),  analysis  of  augite-andesite  fi-om  . . . 

table  following  page  151 
analysis  of  quartz -porphyry  from, 

table  following  page  151 

Analyses,  table  of follows  page  151 

Andesite 374 

iSee  Augite-andesite  and  Homblende-andesite.) 

and  propylite,  Zirkel  on 27 

assays  of 155 

occurrence  of  382 

Apatite.    (>See  illustrations.) 

brown,  in  augite-andesite  64 

brown,  in  earlier  homblende-andesite 56 

brown,  in  eruptive  dioiite 40 

Ascension  theory,    applicability  to  the  Comstock,  von 

Richthqfen  on 19 

Assays  of  Comstock  rocks 154, 223 

of  rocks,  results  of 224 

Attwood,  Gr.,  analysis  by 153 

Augite.     {See  illustrations.) 

conditions  of  crystallization  of 63 

formation  of  chlorite  from 211 

formation  of  pyrite  from 210 

orientation  of  obliquely  cut  twins  of 113 

with  black  borders 123 

with  contorted  twin  lamellfe  129 

with  pinacoidal  cleavage  in  homblende-andesite .     55 
Augite-andesite.    ((See  illustrations.) 

analysis  of table  following  page  151 

description  of  slides  of 126 

feldspars  of 407 

independence  of  eruption  of 202 

lithological  description  of. 62 

occurrence  and  age  of 201 

occurrence  of  ore  in 202 

peculiar  weathering  of. 65 

Bag,  advantages  of,  as  terminal  , 356 

Baltimore,  granite  of  the 34, 190 


414 


GENERAL  INDEX. 


Page. 

C.  &.  G.  (D.  3),  assay  of  diabase  from  the 155 

cross-acction  through  the 269 

diorite  in  the 197 

Calcite,  occnrrence  as  gangne  219 

Calcium  and  magnesium  salts,  relative  solubility  of. 212 

Caledon  ia,  assay  of  granular  diorite  from  the 154 

assay  of  quartz-porphyry  from  the 155 

later  diabase  in  the 199 

metamorphics  in  the 191 

quartz-porphyry  of  the 47 

remarkable  flood  in  the 232 

Calibration  of  thermo- element 298 

Caii/ornia.  analysis  of  ore  from  the 153 

V.  Richthofen  on  the  prospects  of  the 23 

Calif omia  amd  CoTisolidated  Yir^mia  bonanza 270 

ore  of  the 219 

sugar    qnartz    of 

the 221 

Carbonic  acid  as  a  solvent 226 

part  played  by,  on  the  Lode 386 

C  arpath  ian  Mountains,  propylite  of 81 

Cedar  Hill  (D.  2),  analysis  of  diorite  from table  fol- 
lowing page  151 

dioritesof 40,42,97,150,195 

lithological  character  of 15 

ore  deposits  of 1-6 

quartz  veins  of 218,219,220,225 

Cedar  Hill  Canon  (D.  2),  andesites  of 55,56 

earlier  homblende-andesite  from.  34, 122, 201 

Cedar  Hill  veins  part  of  the  Comstock 220 

Central,  von  Kichthofen  on  the  prospects  of  the  23 

Chemistry  of  the  Lode 209,384 

Chimneys  produced  by  faulting 179 

Chlorine,  part  played  by,  on  the  Lode 20, 386 

Chlorite.     {See  illustrations.) 

alteration  of,  to  epidote • 213 

alteration  of  the  quartz  and  carbonates   213 

characteristics  of 369 

decomposition  of 77,  384 

formation  of,  from  ferro-magnesian  silicates    .-   74, 

210, 384 

solubility  and  diffusion  of 40 

OhoUar,  analysis  of  clay  from  the table  following 

page  151 

a.ssay  of  diabase  from  the. 155 

diabase  of  the 116 

diorite  of  the 96 

eastern  fissures  in  the 276 

later  diabase  from  the 151 

Church,  J.  A.,  denies  that  the  Comstock  is  a  vein 30 

diagrams  of  the  Consolidated  Virginiaaiid 

California  bonanza 270 

memoir  on  the  Comstock  Lode 28,29 

on  diorite 184 

on  faulting 156 

on  loss  of  heat  in  the  mines  -  - 239 

on  sugar  quartz 30 

on  the  heat  of  t  ho  Comstock 31, 230, 290 

on  the  Justice  ore  body 30 

on  the  lithology  of  Washoe    28 

on  ventilation  of  the  mines 3 

Circuit,  breaks  in,  how  detected 329 

resistances  of -ISO,  334,  338 

Claim-map,  preparation  i>f -84 

Clays,  analyses  of tablo  following  page  151 

character  of  the 273 


Page. 

Clays,  not  essentially  kaolin 217 

of  the  Comstock 394 

of  the  Lode,  von  Eichthofen  on 17 

Clifford  mine,  "Wales,  temperature  of  the 228 

Coldberry  lead  mine,  electrical  activity  of  the 311 

Collections,  enumeration  of  the 207 

extent  of  the 369 

Ooim^nation  shaft  (D.  4),  assay  of  earlier  homblende- 
andesite  from  near  the 155 

earlier  homblende-andeaite  near  the    56, 
117, 151 
temperature    observations     in 

the 231,244,246,260 

Commutator 328,354 

Comstock  ffssnre  due  to  trachyte  eruption,  von  Eicht- 
hofen on 18 

Comstock  Lode.     (iSee  Vein.) 

age  of 184 

bullion  product  of  the 10 

character    of    the,    below    the    great 

horse 267 

Church's  memoir  on  the 28 

complex  structure  of  the 269 

conclusions  regarding    the  history  of 

the 285 

conclusions  regarding  the,  von  Eicht- 

hofen's 23 

condition  of  the,  during  examination..  266 

contents  of  the 393 

contents  of  the,  von  Eichthofen  on 16 

continuity  in  depth,  von  Eichthofen  ..  21 
cross-section  of  the,  through  the  C.  £  C.  269 
cross-section  of  the,  von  Eichthofen  on .     14 

detailed  description  of  the 266 

dip  of  the,  in  Gold  Hill 277,283 

dynamical  action  in  the,  von    Eicht- 
hofen on  16 

electrical  activity  of  the 317, 321, 322 

faulting  on  the,  von  Eichthofen  on  . . .    15 

general  character  of  the 392 

general  outline  of  the  266 

geographical  position  of  the 2 

history  and  statistics  of  the 368 

history  of  the,  Church  on  the 29 

King's  memoir  on  the 24 

King's  summary  of  the  geologyof  the. .  25 
mean  width  of,  von  Eichthofen  on  ...  21 
mode  of  occurrence,  von  Eichthofen  on  14 
narrowness  of  the,  on  the  Sutro  Tunnel 

level 283 

not  a  vein,  according  to  Church 30 

probabilities  for  its  future 287, 306 

probable  character  in  depth,  von  Eicht- 
hofen on  22 

report  on,  by  von  Eichthofen 12 

summary  of  the  geology  of  the 368 

walls  of  the 267 

Comstock  mines.     (See  Mines.) 

Conclusions,  von  Eichthofen's,  regarding  the  Lode 23 

Conduction  curve,  the  Sutro  TMnnc^  temperatures  form  a.  263 

Consolidated  Virginia,  cross-section  through  the 209 

Consolidated  Tirginiu  and  California  bonanza 270 

electrical    activity 

in  the 317,322 

Contact,  electrical,  with  vein.. 318,324 

permanent - 317 


GENERAL  INDEX. 


415 


Contact,  i)ermanont position  of,  on  400-foot  leTel  of  the 

Eiclimond  mine 337 

position  of,  on  500-footlevel  of  Rich- 
mond mine 334 

position  of,  on  600-foot  leTel  of  Eich- 

mond  mine 344 

position  of,  on  surface  of  Richmond 

mine 51 

temporary 317 

Contact  bag,  description  of 324 

Contacts,  metallic  (plates  and  gada),  objectionable 358 

Cooling,  influence  of,  on  precipitation 226 

Cornwall,  electrical  activity  of  ore  bodies  in 310,311,312 

Cortez  Peak,  propylito  from 142 

Range,  foothills  of,  propylite  from 140 

Councler,  C,  analysis  by table  following  151 

Country  rock  attacked  by  acids 226 

Cross-spur  below  graveyard,  analysis  of  earlier  horn- 
blende-andesite  from  - . . 
table  following  page  151 

■propylite  from li'Z 

Cross-spur  quarry  (D.  2),  analysis  of  later  homblende-an- 

desite  from table  following  page  151 

Crown  PointRavine  (B.4),  angite-andesite from. 87, 126,128, 151 
earlier  hornblende- andesite  from.  .87, 125 

epidote  from 213 

propylite  from 86, 87, 135, 136, 137 

Crushing  action,  eflects  of 394 

Curtis,  J.  S. ,  assays  of  rocks  by 155, 223 

Dayton,  subaqueous  eruptions  near 14 

Decomposition.    {See  illustrations.) 

area  of 72, 238, 383 

due  to  aolfataric  action 240 

evidence  of  an  external  cause  for  the. ..  239 

of  blocks  of  rock 79 

of  minerals.     (See  each  mineral.) 

of  the  rocks 72, 209, 369 

((SeeUthological  description  of  each  rock. ) 

rocks  affected  by 239 

Devil's  Gate  (D.  6),  raetamorphic  diorite  near 108 

Dewey,  F.  P.,  analysis  by 154 

on  the  feldspars  of  the  later  homblende- 

andesite 68 

Diabase,  assays  of -.154, 155 

earlier.    (See  illustrations.) 

analysis  of table  following  151 

description  of  slides  of 112 

feldspars  of 

homblendic 52 

laminated 51 

lithological  description  of 48 

occurrence  and  age 197, 374, 381 

relations  to  the  Lode 197 

silica  contents  of 152 

later.     (See  Black  Dike.) 

description  of  slides  of 116 

lithological  description  of 52 

occurrence  and  age  of 198, 381 

silica  contents  of 152 

of  Ophir  Ravine,  possibly  a  diorite 36 

of  Orange  Mountain,  N.  J 53 

selected  for  experiments  on  kaolinization 236 

silver  traced  to 224 

the  possible  source  of  ore 222 

Diorite,  assays  of 154, 155 


Diorite,  eruptive 378 

(See  illustrations.) 

brecciated 41 

containing  tourmaline 97 

dark  varieties 38 

feldspars  of 406 

granular,  analysis  of table  following 

page  151 

granular,  description  of  slides  of 93 

hypothesis  concerning 194 

lithological  character  of 34 

micaceous,  analysis  of table  follow- 
ing page  151 
micaceous,  occurrence  in  the  Yellow 

Jacket 277 

micaceous,  porphyritic,  description  of 

slides  of 104 

occurrence  and  age 192, 381 

porphyritic 39 

porphyritic,  analysis  of . .  .table  follow- 
ing page  151 
porphyritic,  description  of  slides  of  ..    97 

porphyritic,  laminated 41 

porphyritic,  silica  contents  of 152 

possibility  of  metamorphic  origin 195 

relations  of  varieties  of 42, 193 

metamorphic,  analysis  of table  following  page  151 

brecciated 43 

descriptions  of  slides  of 106 

feldspars  of 406 

lithological  character  of 43 

occurrence  and  age 195 

relations  to  quartz-porphyry 43 

Zirkel  on 12 

Dcelter,  C,  on  propylite 90 

Drill-holes,  disposition  of 325 

without  specific  electrical  effect 360 

Dutton,  C.  E.,  on  propylite 81 

Earth-currents,  intensities  of. 346 

discrepancies  produced  by 360 

Earth-potential,  constancy  of 352 

graphic  representation  of 342, 348, 354, 

362. 366 

how  measured 320, 327, 328, 352 

maximal  values  of 317 

observed  at  the  Richmond  mine 340,  347, 

350,  351,  355 

series  of  values  of 329 

values  of  on  400  and  500-foot  levels  of 

the  Richmond  mine 342 

values  on  600-foot  level  of  the  Richmond 

mine 349 

variation  of,  on  the  surface 351 

variation  of,  with  distance 342,  348, 352 

East  vein,  a  result  of  faulting 182 

inferences  from  the 271 

Eckart,  W.  R.,  mining  machinery  of  the  Comstock 1 

Eldorado  croppings,  analysis  of  diorite  from  the table 

following  page  151 

diorite  dike  near  the 40, 42, 104 

Electric  survey  across  au  ore  body 344 

over  the  surface 351 

through  an  ore  body  (Eureka) 332 

Electrical  action,  confined  to  particular  parts  of  the  ore 
body 351,364 


416 


GENERAL   INDEX. 


Page. 
Electrical  action,  local  effect  of,  graphically  represented .  367 

Electrical  activity  of  ore  bodies 309 

of  ore  bodies,  causes  of.310, 311, 312,  313, 314 
of  ore  bodies,  method  employed  in  de- 
termining   402, 403 

of  ore  bodies,  results 403 

of  the  Consolidated  Virginia  Rnd  Cali- 

fomia  319, 322 

of  the  Mexican 322 

of  tho  Ophir 322 

of  veins,  earlier  work  on 313 

Electrical  difference  between  zinc  strips  of  terminals 357 

Electricity  of  metallifereous  veins,  Foxonthe-309,  310,  311,  312 
Hen  wood  on  the. .  .309, 311 
Keich  on  the  ..309, 312, 313 
von     Strom  beck     on 

the 309,310 

Electrometer  for  measuring  earth-potential 353, 367 

Electromotive  force,  how  measured 295 

zinc,  earth,  copper 323 

Empire- Jmperiai,  analysis  of  water  from  the 152 

Energy,  logarithmic  distribution  of,  in  taulting    162 

transmission  of,  by  friction 159 

Epidote.  {jSee  illustrations.) 

circumstances  favoring  the  formation  of 21] 

_  derived  from  chlorite 75, 213,  370, 384 

insolubility  of 78 

mode  of  occuiTence  of 212 

not  formed  at  the  expense  of  feldspar 76, 216 

Equipotentials,  contour  and  position  of  the,  relative  to  the 

ore  bodies 365 

surrounding  the  ore  body 315 

Erosion  in  Washoe 285 

Errors  in  the  electrical  results  for  the  400  and  500-foot 

levels,  Richmond  mine 343 

Eurr'ka  District,  electrical  activity  of 324 

Evaporation  as  a  cause  of  precipitation 227 

E.-qieriments  on  electrical  activity  of  veins,  hypothesis 

underlying 314 

Fault,  as  shown  on  the  Suiro  Tunnel  level 282 

diminution  of,  near  ends  of  Lode 185 

on  the  Lode,  probably  caused  by  rise  of  the  west 

country 185 

on  the  TItak  section 281 

period  of  the 272 

to  the  north,  in  the  Sierra  Nevada 281 

Fault-curve,  angle  which  tangentmakes  with  theborizon  172 

equations  of 161, 163, 164, 165, 180 

point  ofminimum  radius  of,  curvature  of  ...  171 
reduction  and  interpretation  of  the  equation 

for..*. 168 

where  two  or  more  rocks  are  involved 172 

with  double  cnrvature 173 

Faulted  surface,  character  of 177 

Faulting 285 

accompanied  by  parallel  fissuring 174 

Church  on 156 

evidences  of 157 

experiments  on 165 

influence  on  the  width  of  the  Lode  on  the  Belcher 

section 278 

King  on 156 

on  the  Union  shaft  section 279 

atnictural  results  of 156,377 

theory  of,  application  to  landslips 390 


Page. 

Faulting,  theory  of,  application  to  the  Gomatook,  and 

other  instances 179 

theory  of,  application  to  veins 379 

von  Eichthofen  on 156 

Feldspars,  decomposition  of 78, 215, 271, 385 

determined  by  Szab(3'8  method 405 

external  energy  involved  ia  the  decomposi- 
tion of , . 234 

inferences  concerning  composition  of 407 

of  later  homblende-andesite,  analyses  of . . .  155 

representative  composition  of 233 

Ferro-magnesian  silicates,  decomposition  of 384 

Fissure,  the  Comatock,  due  to    trachyte   eruption,  von 

Richthofen  on jg 

Fissures  in  hanging  wall  produced  by  faulting 178 

in  the  Yellow  Jacket  dipping  west 277 

Floods  supposed  to  have  induced  kaolinization 232 

Florida,  metamorphics  near  the 190 

FloweryPeak(F.  2),  later  homblende-andesite  from  near  133 

Flowery  Range  (F.  2),  erosion  on  the 204 

Fluorine,  instrumentality  of,  on  the  Gomstock,  von  Richt- 
hofen ou 20, 380 

Foot  walls,  rise  of,  in  faulting 378 

i^orman  shaft  (D.  5),  assay  of  augite-andesite  from  near 

the -7 155 

augite-andesite  at  the 30, 200, 202 

cross-section  through  the 278 

earlier  homblende-andesite  at  the . . .  200 
equation  between  temperature  and 

depth  in  the 262 

eruptive  diorite  near  the 192 

qnartz-poi-phyry  of  the 47,196 

temperature  observations  in  the 231, 

244,  252,  260 
Fox,   experiments  on    the  electricity  of  metalliferous 

veins  309, 310, 311, 312 

Freiberg,  electrical  activity  at 312,  313 

Friction,  coefficient  of,  of  Gomstock  rocks 186 

part  it  plays  in  faulting 158 

Reuleaux  on  158 

Fuel,  consumption  of,  by  the  mills 6 

transportation  of , 6 

Gaobro-like  diabase 115 

Gads,  steel  and  copper,  electromotive  force  and  polariza- 
tion of 318, 319 

G-alleries,  mine,  length  of 5 

Galvanometer » 298,  320, 327 

Gate  of  Mimroe,  propylite  from 144 

Geiger  grade  toll-house  (D.  1),  1,200  feet  north  west  of  the, 

earlier  homblende-andesite  from 120, 121, 406 

Golconda  station,  propylite  from 141 

Gold  and  silver,  proportions  of,  in  Gomstock  bullion  .  .0, 9, 18 

Gold  detected  in  "Washoe  rocks 223    . 

Gold  Hill  and  Virginia,  population  of 4 

Gold  Hill  mines,  2,500-foot  level  of  the 284 

Gold  Hill  Peak  (c.  4) ,  propylite  from 86, 87, 137 

GovXd  (£  Curry  and  Savage,  bonanzas  of 17 

Goupilli6re,  Haton  de  la,  on  exploitation  of  mines 309 

Granite,  assay  of 154 

description  of  slides  of 91 

feldspars  of 406 

lithological  description  of 34, 373 

occurrence  and  age  of 190,380 

Great  Basin,  character  of 2 

Greenstone,  meaning  of  the  term 376 


GENERAL  INDEX. 


417 


Page. 

Grunow,  "W.,  galvanometer  made  by 208,327 

HabitnB,  value  of,  in  rock  determinationB 85 

Hague,  Arnold,  analysis  by 153 

Hagae,  J.  D.,  on  the  system  of  timbering  in  the  mines. .      6 

Hale  dk  Norcross,  analysis  of  clay  from  the 

table  following  page  151 

analysis  of  water  from  the 152 

cross-section  through  the 276 

flood  in  the 232 

Hanging  wall,  rise  of,  a  rare  occurrence 179, 378 

Hawes,  G.  "W.,  on  the  feldspars  of  the  later  hornblende- 

andesite 67 

Heat.    (See  Temperature.) 

casualties  from,  in  the  mines 3 

distribution  of 230 

loss  of,  in  the  mines 229 

nonnal  increment  of 229 

of  the  Comstock,  Church  on  the 31 

depth  of  the  source  of  the.  be- 
low the  surface 240 

explanations  offered 231 

mines 3 

relations  to  surface  rocka 241 

results  of  thermal  survey  con- 
cerning   264 

phenomena  of  the  lode 228, 387 

Heinrich,  F.,  on  the  Siwrenberg  boring 245 

Hen  wood,  W.  J. ,  on  electricity  of  metalliferous  veins . .  309, 31 1 

History  of  mining  on  the  Comstock,  by  E.  Lord 1 

History  of  the  Lode,  Church  on  the 29 

Hoffmann  &  Craven,  mapping  by 284 

Holzappel,  v.  Strombeck's  experiments  at 31 0 

Hornblende.     (See  illustrations.) 

Hornblende,  black-bordered.    (See  illustrations.) 

characteristic  of  andesite  . .    84 

brown,  alteration  to  green 36 

decomposition  of 74 

disseminated  through  groundmassof  ande- 
site      84 

formation  of  chlorite  from 211 

formation  of  pyrite  from 210 

green,  fibrous,  chlorite  mistaken  for 84 

green,  resulting  from  alteration  of  brown . .    41 

speculation  on  the  black  border  of 59 

with  double  black  border 54, 123 

with  inclusions,  probably  of  ilmenite 95, 98 

Homblende-andesite,  earlier.    (See  illustrations.) 

analysis  of table  fol- 
lowing page  151 
containing  finely  dissem- 
inated hornblende 120 

containing  ilmenite 118 

description  of  slides  of . .  116 

feldspars  of 406 

lithological  description  of    53 

occurrence  and  age 199 

occurrence  of  ore  in  . . .     201 
with  disseminated  horn- 
blende      54 

with  excess  of  augite  ...     55 
with    very   large   horn- 
blendes   57,201 

later.    (See  illustrations.) 

analysis  of table  fol- 
lowing page  151 

27  0  L 


Page. 

Homblende*andesite,  later,   description  of  slides  of 130 

determination  of 383 

Dewey  on  the  feldspars  of.    68 

feldspars  of 153 

Hawes  on  the  feldspars  of     67 
lithological  description  of. .    66 

occurrence  and  age  of 203 

slight  erosion  of 203 

Zirkel  on  a  feldspar  In 69 

Horse,  the  great 267 

Horses,  large,  characteristic  of  upper  levels,  von  Kicht- 

hofen  on 22 

near  surface,  von  Richthofen  on 16 

produced  by  faulting 179 

Hydrosulphuric  acid  in  YeUow  Jacket  water 240 

Ilmenite.     (See  illustrations.) 

decomposition  of 215 

in  earlier  homblende-andesite 56,118, 123 

lithological  description  of 145 

probable  acicular  inclusions  of,  in  hornblende  38, 40 

Imperial,  diorite  of  ravine  (C.  4.),  west  of  the 41 

Inclusions,  black,  acicular.     (jS'ee  illustrations.) 

fluid,  as  diagnostic  points 50 

containing  salt  cubes  in  eruptive  diorite    37 

in  homblende-andesite 121 

secondary.    (See  illustrations.) 

secondary 79, 119, 371 

glass.    (See  illustrations.) 

double.     (See  illustrations.) 

occurrence  in  propylite 85 

Intensities,  constancy  of 341, 348 

observed  on  the  400-foot  level  of  Kichmoud 

mine 338 

observed  on  the  500-foot  level  of  Kichmond 

mine 336 

Investigation,  previous,  of  the  Comstock  Lode 12 

Johnson,  S.W.,  analysis  by table  following  page  151 

Julia,  assay  of  diabase  from  the 155 

earlier  diabase  near  the 197 

flood  in  the 232 

Justice,  bonanza  of  the 394 

Church  on  the 30 

croppings  near  the 278 

eruptive  diorite  in  the 193 

metamorphic  diorite  of  the 196 

ore  bodies,  probably  of  same  age  as  the  others  . .     220 

ore  of  the 219,  220, 225 

quartz-porphyry  near  the 33 

Kaolin,  a  hydrate 235 

heat  of  hydration  of,  unknown  ^ 235 

not  identified  in  the  rocks 216 

not  prevalent  on  the  Lode 78, 371 

reactions  leading  to  formation  of 234 

Kaolinization,  criticism  of  the  evidence  that  heat  is  pro- 

duced  by 232 

discussion  of  experiments  on 304 

discussion  of  the  theory  of 233 

disposition  of  apparatus  for 295 

errors  in  experiments  on 308 

evidences  of  heat  caused  by 231 

experimental  results  discussed 304 

experiments  on 236 

general  plan  of  experiments 290 

heat  evolved  by 235 

heat  of  the  Lode  attributed  to 231 


418 


GENERAL  INDEX. 


Page. 

Kaolinization  hypothesis 388 

hypothesis,  conclusions  regarding  the 237 

not  prevalent  at  Washoe 218,  '237 

results  obtained  from  experiments  on 304, 

306,  307.  399 

results  of,  final  expression 307 

thermal  eflfect  of 290,397 

Karpathian  Mountains,  propylitic  character  of  the 13 

Kentuck,  analysis  of  ore  from  the 153 

King,  Clarence,  memoir  on  the  Comstflck  Lode 24 

on  fanlting 156 

on  propylite 81 

on  quartz-propylite 13 

on  the  rela^tions  of  propylite  and  ande- 

sit« 24 

on  the  structure  of  bonanzas 271 

on  the  west  wall  of  the  Comstock 25 

summary  of  the  geology  of  the  Com- 
stock   = 25 

Kittlcr,  E.,  diflerence  of  potential  of  liquids  in  contact..  358 

Kormann,  W.,  analysis  by table  following  page  151 

Lady  Bryan,  eruptive  diorite  in  the 192 

Lamination  of  country  rock 182 

Landslips,  theory  of 187,380 

Lateral  secretion  theory 225, 385, 396 

Lawson'g  Tunnel  (B.  5),  quartz-porphyry  1,000  fe«t  south 

of 108,150 

Leucoxene,  occurrence  of '. 215 

Litbology.    {See  Rocks.) 

importance  to  theory  of  ore-deposits 32 

of  the  Washoe  District 32 

of  "Washoe,  Church  on  the 28 

summary  of 369 

Tx)de,  Comstock.    (See  Vein.) 
Lode.    (See  Comstock  Lode.) 

the  Comstock,  detailed  description  of 266 

Lode  currents,  hypothesis  underlying  experiments  on. . .  314 
opportunities  forinvestigating  at  Eureka.  324 

Lode  electromotive  force,  bow  measured 320,  327 

Lord,  Eliot,  history  of  mining  on  the  Comstock 1 

on  the  product  of  the  Lode 7 

Machinery,  mining,  of  the  Comstock,  by  W.  R.  Eckart. . .      1 

Magnetite,  secondary 214 

Mallet,  R.,  theory  of  teirestrial  heat  applied  to  Washoe.  231 

Matteuci,  Ch.,  on  earth  currents 352, 360, 367 

McClellan  Peak  (6  west),  basalt  near 71 

MeKibben  Tunnel,  assay  of  granular  diorite  from 154 

diorite  of  the 33, 39, 42, 75, 76, 87,  99, 102 

epidote  in  the 213 

rocks  of  the 28, 87 

sheeted  structure  in  the 183 

Uetamorphic  rocks,  Whitney  on  the  age  of 13 

Metamorphics  in  the  Sierra  I^evada 280 

occurrence  and  age  of 190,  380 

Method  of  least  squares,  application  to  temperatures 245 

Mexican^  electrical  activity  of 317,322 

tenor  of  ores  of 18 

Mica.    (See  illustrations.) 

Mica,  biaxial 130,131 

formation  of  chlorite  from 211 

Milling 6 

of  slimes  and  tailings 6 

process  used 6 

returns  guaranteed 6 

Mills,  table  of  supplies  used  by  the,  in  1879 8 


Page. 

Mine  maps 284 

Mines,  bullion  product  of  the 10,11 

development  of  the,  in  1865 14 

extent  of  the 5 

heat  of  the 3 

importance  of  the 1 

length  of  gaUerics 5 

system  of  timbering  in  the 5 

table  of  supplies  used  in  1879 8 

the  Comstock l 

ventilation  of  the.  Church  on 3 

Mineral  belts  west  of  the  Rocky  Mountains 2 

Miners,  average  weight  of 4 

good  condition  of 4 

hours  of  labor  of 3 

number  employed 4 

wages  of - 4 

work  performed  by 4 

Mining,  commencement  of,  on  the  Comstock 2 

difficulties  of,  on  the  Comstock 2 

Mister,  W.  G.,  analyses  by table  following  page  151, 153 

Moisture  in  the  rocks,  electrical  effects  of 356 

Montezuma  Range,  propylite  from 139 

Moore,  G.  E.,  analysis  by table  following  page  151 

Mount  Abbie  (C.  2),  later  homblende-andesite  of.  .70, 133,  205 

Mount  Butler  (B.  4),  lithological  character  of 15 

Mount  Davidson  (C.  3),  age  of 89 

diorites  of 33, 35, 42, 88. 193 

infiuence  on  the  Lode 156 

King  on 2£ 

probably  an  uplift 18£ 

propylite  from 143 

sheeted  structure  of 182 

slope  of 377 

von  Richthofen  on 13, 14, 15 

Mount  Button,  Utah,  propylite  from 143 

Mount  Emma  (G.  4),  younger  hornblende-andesite  of. . .      69 
and  Mount  Rose,  divide  between,  later 

hornblende-andesite  from 134 

Mount  Kate  (F.  5),  augite-andesite  from 127 

Mount  Rose  (F.  5), analysis  of  later  homblende-andesite 

from table  following  page  151 

later  hornblende  -andesite  of 69, 153, 204 

Munroe.Gateof,  Utah,  propylite  from 144 

Occidental  grade,  augite-andesite  of  the 34 

Lode,  occurrence  of 202 

Openings,  lenticular,  infrequency  of,  on  the  Comstock . . .  395 

Ophir,  analysis  of  ore  from  the 153 

analysis  of  water  from  the 152 

assay  of  porphyritic  diorite  from  the 1.^4 

electrical  activity  in  the 317,322 

tenor  of  ore  of  the 18 

Ophir  grade,  augite-andesite  of 128, 407 

Ophir  Hill  (B.  3),  earlier  homblende-andesite  near 116 

Ophir  Ravine  (C.  2),  assay  of  porphyritic  diorite  from...  154 

diabase  of 51 

diorite  of 33,  36,  83,  98, 102, 103, 152, 406 

epidote  in 213 

propyUteof S2,83,86,I3H 

Orange  Mountain,  N.  J.,  diabase  of 53 

Ore.    (See  Bonanzas.) 

Ore  and  quartz,  origin  of 221 

bodies,  cause  of  electrical  activity  of,  on  the  Com- 
stock   323 

electrical  activity  of 309,400 


GENERAL  INDEX. 


419 


Oro  boilies,  likely  to  bo  found  in  the  hanging  wall,  von 

Richthofenon 

Uepoaits,  magnetic  efifects  of 309, 

depoaitiou  of 

distribution  of 

found  on  the  west  face  of  the  diabase 

influence  of  the  rocks  on  the 

minevala  of  the  Comstock   270, 

near  the  surface  on  the  Union  shaft  section  

occurrence  of,  on  the  Comstock 

precipitation  of,  from  solution 

probabilities  of  finding  at  great  depths 

probable  cause  of  fluctuations  in  the  tenor  of  the 

relation  of  the,  to  tlie  rocks 218, 

small  amoimt  of,  in  sight  during  examination 

source  of,  relations  of  decomposition  to 

source  of,  von  Kichthofeu  on 

tenor  of 

Ores,  analyses  of 

earthy  character  of,  at  Eureka 

electrical  properties  of 311, 

how  dissolved 

of  diflerent  grades,  time  relations  of 

of  the  Lode,  von  Richthofen  on 

uniform  character  of,  von  Richthofen  on 

jield  of 

yield  of,  von  Richthofen  on 

Osbiston  shaft  (D.  3),  diabase  in  the 

Outer  liquid 

Overman,  assay  of  diabase  from  the 

assay  of  porphyritic  diorite  from  the 

earlier  diabase  in  the 

later  diabase  in  the  

partial  cross-section  through,  the 

quartz -porphyry  of  the 47, 110, 

Plagioclase  with  zonal  structure  in  eruptive  diorite 

zonal  structure  of.     (See  illustrations.) 

Population  of  Virginia,  Gold  Hill,  and  Silver  City 

Potential,  continuity  in  variation  of,  with  distance. .  .354, 

ditference  of,  between  terminals 

Precipitation  accelerated  by  evaporation 

conditions  affecting 226, 

Product,  bullion,  from  tailings 

of  the  Comstock  Lode,  by  mines,  table. 

of  the  Lode,  Lord  on ■ 

Propylite 81, 

{See  illustrations.) 

analysis  of table  following  page 

and  andesite,  Zirkel  on 

association  with  silver  ores,  von  Richthofen  on 

causes  leading  to  its  determination 

European 

'  from  other  districts  thau  Washoe 

from  Utah,  descriptions  of  slides  of 

King  on  its  relations  to  andesite 

typical  localities  of 

von  Richthofen  on 

von  Richthofen's,  based  largely  on  Washoe  oc- 
currences   

Zirkel's  diagnostic   points  baaed  largely  on 

Washoe  occurrences 

Propy  lites  of  the  Fortieth  Parallel,  description  of  slides  of 

Prospecting,  rules  applicable  to 

Pumpelly,  R.,  on  rock-weathering 

Pyrite,  fonnation  of 75, 

heat  of  the  Lode  attributed  to  tbe  oxidation  of. . 


Page. 

Pyrite,  relations  of,  to  ore 222 

relations  to  the  ferro-magnesian  silicates 210 

Quarry  1,000  feet  west  of  the  Tellmv  Jacket  ed&t  shaft 

(C.  4),  earlier  hornblende-andesite  of  the 119 

500  feet  N.  of  N.  Twin  Peak  (C.  4),  andesite  of 34 

1,500  feet  SW.  of  Justice  (C.  5),  assay  of  quartz- 

porpiiy  ry  from 155 

2,000  feet  E.  of  Occidental  Mill  (E.  5),  later  horn- 
blende-andesite of: 34,  67,  69, 131 ,  407 

2,000  feet  NE.  of  Sutro    shaft  III.,  later  horn- 
blende-andesite from 34,  66,  69, 130, 151,  407 

2,000  feet  NE.  of  Sutro  shaft  III.  (E.  4),  assay  of 

later  hornblende-andesite  from 155 

near  the  Sierra  Nevada    {D.  2),  younger  horn- 
blende-andesite of 70 

near  the  Utah  (D.  2),  assay  of  later  hornblende- 
andesite  from 155 

near  the  Utah,  later  hornblende-andesite  from ...    34, 

131,  407 

Quartz  and  ore,  origin  of 221 

crushed,  von  Richthofen  on 16 

deposited  in  openings • 273 

diflerence  between  east  and  west,  von  Richthofen 

on 16 

gold,  on  Cedar  Hill,  von  Richthofen  on 16 

how  dissolved 226 

in  earlier  hornblende-andesite 121 

occurrence  of  solid  and  of  crushed 270 

precipitation  of,  from  solution 226 

secondary,  characteristics  of 85 

sugar.  Church  on 30 

von  Richthofen  on 17 

Quartz -poi-pbyry 373 

(See  illustrations.) 

analysis  of table  following  page  151 

assays  of 155 

decomposition  of 79 

description  of  slides  of 108 

feldspars  of 1 10, 406 

felsitic  variety  of 47 

lithological  description  of 45 

occurrence  and  age  of 196 

separation  of,  by  Tboulet's  method 110 

von  Richthofen  on  the 13 

Quartz-propy lite.  King  on 13 

Rath,  G.  vom,  on  propylite 90 

Ravines  produced  by  faulting 177 

Keade,  F.,  computations  by 245 

Red  Jacket,  assay  of  granite  from  the 154 

granite  of  the 33,34,91,190,405 

Reich,  F.,  experiments  on  the  electricity  of  metalliferous 

veins 309,312,313,317,365 

Resistance  of  circuits 330,  334, 338,  345,  351 

of  layers  of  rock  between  consecutive  similar 

surfaces 359 

of  rocks  decreases  with  porosity  and  moist- 
ure  341,348,351 

specific,  of  rock  in  place 359 

Results  for  earth-potential  from  different  methods 355 

Reuleaux,  F.,  on  ftiction 158 

Richmond  mine  chambers,  Nos.  7, 10, 11,  12, 13, 14,  15, 16, 

etc 332 

Nos.  14  and  15  connected  elec- 
trically   350 

disposition  of  points,  500-foot  level 335 

disposition  of  point.s,  400-foot  level 338 


420 


GENEEAL  INDEX. 


Page. 

Richmond  mine,  electrical  activity  of 324 

plau  of  drifts  and  workings,  400  and  500- 
foot  levels 333 

plan  of   drifts  and  workings,  600-foot 

level 344 

Kichinond  mine  ore  bodies,  extent,  horizontal,  of 332 

relative  position  of  the 331 

Kichthofen,  F.von,  conclusioos  as  to  the  Comstock 23 

estimated  yield  of  the  Lode  to  the 

close  of  1865 9 

his  predictions  verified 12 

on  erosion 271 

on  faulting   156 

on  fluorine  and  chloi-ine 20, 386 

on  propylite 81 

on  the  alteration  of  minerals  in  situ. .    20 
on  the  applicability  of  the  ascension- 
theory 19 

on  the  contents  of  the  Lode 16 

on  the  continuity  of  the  Lode  in  depth.    21 

on  the  east  vein 182 

on  the  mode  of  occurrence  of  the  Com- 
stock     14 

on  the  probable  character  of  the  Lode 

in  depth 22 

on  the  proportion  of  gold  to  silver  in 

Comstock  bullion 7 

on  the  quartz-porphyry 47 

on  the  rocks  of  the  Washoe  District.    12 

on  the  source  of  ore  18 

on  widespread  solfataric  action 21 

report  on  the  Comstock  Lode ,.    12 

ilickard,  E.,  data  concerning  the  Eureka  ore  bodies 331 

W.  F.,  analysis  by 153 

Kock  subjected  to  action  of  aqueoas  vapor,  description 

of 299 

Kocks.     (See  Lithology.) 

assays  of 154 

conductivity  of 341,  348,  351 

eruptive,  means  of  determining  snccession  of 188 

general  character  of  the  decomposition  of  the  . . .  209 

metallic  contents  of 223 

moisture  of  the 241 

occurrence  and  succession  of  the 188, 380 

of  the  Distiict,  typical  character  of  the 374 

of  the  Washoe  District 32,372 

of  the  Washoe  District,  Zirkel  on  the 26 

special  localities  of,  in  the  Washoe  District 33 

their  relations  to  ore-deposits 32 

Washoe,  disputed  character  of  the 33 

which  contain  silver  and  gold 225 

Hock-chamber 293 

difference  of  temperature  of  outside  and 

_  interior  of 300, 302, 303 

Sock  laland,  granite  at  the 34, 190 

metamorpbics  in  the 191 

Rock-maases,  concentric  weathering  of 371 

Rose  Bridge  Colliery,  temperature  observations  in  the  .245, 254 

Rosenbusch,  H.,  on  propylite 90 

Roux's  ranch  (C.  5),  assay  of  basalt  from  near 155 

basalt  near 33 

felsitic  quartz-porphyry  near 33 

Sandberger,  F.,  on  the  lateral-secretion  theory 385 

on  the  metallic  contents  of  rooks 221 


Page. 

Savage,  analysis  of  clay  from  the table  following 

page  151 

analysis  of  oro  from  the 153 

analysis  of  water  from  the 152 

assay  of  porphyritic  diorite  from  the 154 

diorite  of  the 96 

flood  in  the 232 

later  diabase  in  the 199 

Savage  and  Gould  t£  Curry,  bonanzas  of  the 17 

Savage  shaft,  cross-section  through  the 274 

Schemnitz,  propylite  of 90 

School  statistics  of  Storey  County 5 

Scorpion  croppings  (E.  2),  position  of  the 193,279 

Shale  in  tbe  Richmond  mine 334,  345 

Shafts,  temperatures  in  various 391 

Sheep  Corral  Cafion,  propylite  from 138 

Sheeted  structure  of  country  rock  discussed 182 

on  the  C.  d  0.  section 271 

Sierra  Nevada,  assay  of  diabase  from  the 155 

cross-section  through  the  ..  - 280 

earlier  diabase  in  the 115, 198 

eruptive  diorite  in  the 99, 192 

later  hornblende- andesite  near  the 204 

limestone  in  the 192 

Sierra  Nevada  Range,  water  from  the 243 

Silica  determinations  of  rocks 152 

Silicates,  ferro-magnesian,  decomposition  of  the 214,369 

Silver  and  gold,  distribution  of,  in  the  Comstock 268 

in  rocks  compared  with  yield 224 

proportions  of,  in  Comstock  bullion . .  .6, 9, 18 

Silver  City,  basalt  just  west  of 134 

railroad  (C.  7),   earlier   homblende-andesite 

from 123 

veins  in  homblende-andesite  near 201 

Silver  Hill,  eruptive  diorite  in  the 192 

near  tbe 104 

metamorphic  diorite  of  tbe 196 

Silver  Terrace  (E.  3),  analysis  of  augite-andesite  from 

table  following  page  151 

Silver  traced  to  the  augite  of  diabase 224 

Slate,  assays  of 155 

Slides,  detailed  description  of 91 

method  of  reference  to .  145 

SodaUte  in  granite 34,92 

Solfataraa,  von  Richthofen  on 19 

Solfataric  action,  age  of  the 206 

heat  ascribed  to 237.389 

widespread,  in  the  Washoe  District,  von 

Richthofen  on 21 

Solfataric  gases,  part  played  by,  on  the  Lode 386 

Solutions  in  contact,  electromotive  force  of 357 

saline,  in  contact  holes 357 

Skeers  lead  mine,  electrical  activity  of 311 

Sperenberg  boring,  temperature  observations  in  the. -245,  256 
Sphene.    {See  Umenite  and  Titanite.) 

Statistics  of  mining  in  preparation  by  tbe  Census 1 

school,  of  Storey  County 5 

Steamboat  Springs,  solfataric  gases  of 240 

Valley,  propylite  from 138 

Storey  County,  school  statistics  of 5 

Storm  Cauon,  propylite  from - 84,139 

Stratification,  eruptive 182 

Stretch,  R.  H.,  mapping  by 284 

Stringers  from  the  Lode 220 


GENERAL  INDEX. 


421 


Page. 
Stiombeck,  A.  von,  on  eleotrioity  of  metalliferouB  veins, 

309, 310 

Substitution,  theory  of 387 

Snccession  of  eruptive  rocks,  means  of  determining 188 

Sugar  Loaf  Mountain  {F.  3),  later  homblende-andeaite  of     70 
lithological  character  of  . .  14, 394 

Sugar  quartz,  origin  of  the 272 

Sulphydric  acid  in  water  from  the  Yellow  Jacket  240 

Sulphurets,  formation  in  the  vein,  von  Richthofen  on. . .     20 

Solphuric  acid  as  solvent 226, 386 

Summary 36ft 

Supplies,  table  of,  brought  to  the  Lode  in  1879 8 

used  by  the  mines  and  mills  in  1879...      g 

Surface,  electrical  survey  over 351 

Survey,  thermal.    {See  Thermal  survey.) 

Sutro  road,  earlier  homblende-andesite  from  the 124 

Sutro  Tunnel,  air  shaft,  augite-andesite  near 129 

analysis  of  earlier  diabase  from  the 

table  following  page  151 

assay  of  diabase  from  the -.154, 155 

assays  of  rock  from  the  223 

augite-andesite  from  the 127, 129, 201, 202 

cross-section  through  the 274 

earlier  diabase  from  the 

33, 112, 114, 115, 151, 152. 107,  406 
earlierhomblende-andesitefromthe.54, 124, 199 

epidote  in  the 212 

eruptive  diorite  in  the 105, 192 

experiments  on  kaolinization  of  diabase 

from  the 236 

later  hornblende-andesite  of  the 24, 203, 205 

laterals,  temperatures  in  261 

level,  horizontal  section  on  the 281 

logarithmic  character  of  section  on 180 

temperature  curve  influences  from  the 263 

temperature  observations  in  the 231, 244, 

258,  260,  392 

Syenite,  von  Kichthofen  on 12 

Zirkel  on  the  supposed 12 

Szab6,  J.,  feldspars  determined  by  method  of 405 

on  propylite 90 

Table  of  analyses follows  page  151 

analyses 153, 152 

assays 154 

school  attendance  in  Storey  County 5 

supplies  brought  to  the  Lode  in  1879 8 

supplies  used  by  the  mines  and  mills  in  1879 8 

the  bullion  product  from  tailings 11 

the  bullion  product  of  other  mines  in  the  District    II 

the  bullion  product  of  the  Lode 10 

Tailings,  b»Uion  product  from 11 

Temperature,  difficulties  in  obtaining  mean 229 

disturbing  influences  affecting,  in  mines..  220 

equation  between  depth  and 244, 258 

in  the  Sutro  Tunnel,  equation  between  dis- 
tance from  the  Lode 

and 245 

in  dependent  of  surface 

radiation 260 

measurement  of  small  increments  of. .  .291, 295 
(Sea  Heat.) 

normal  increment  of 229 

observations 231,  244 

in  the  Sutro  Tunnel  laterals. .  261 
resultsfrom 201 


Page. 
Temperature  equations,    agreement  between,    for  the 

shafts  and  the  tunnel 263 

high,  of  the  mines 228 

in  mine  workings  not  accordant 230 

reasons  for  some  fluctuations 260 

source  of  high,  on  the  Lode 264 

Terminals,  description  of 318,324 

electromotive  force  between 326 

polarization  of 323 

Thalen,  R..  magnetic  indications  of  deposits  of  iron  ore.  309 

Theory  of  lateral  secretion  affirmed 225 

Thermal  survey 244,391 

conclusions  from  the 264 

results  of,  independent  of  accurate  ther- 
mometers   264 

Thermo-electromotive  force,  correction  for  extraneous 

effects 297 

Themio-element 294 

calibration  of 298,  304 

constants  of 299,  300,  301,  302, 303,  304 

Thermometers,  results  of  thermal  survey  independent  of 

accurate 264 

Thomson,  SirW.,  increment  of  temperature  adopted  by.  229 

Timber,  consumption  of,  in  the  mines 6 

source  of  supply  on  the  Comstock 3 

Timbering,  J.  D.  Hague  on  the  system  of 5, 6 

Titanite  possibly  identical  with  lencoxene 215 

Topography  r.ear  the  Lode  a  result  of  faulting 181 

Tounnaline  in  eruptive  diorite 35 

in  metamorphic  diorite 44, 108 

Trachyte,  sanidin,  von  Richthofen  on 13 

Ti-ansformations  in  the  vein,  chemical,  von  Richthofen  on .     20 

Transportation,  means  of,  to  the  Comstock 3 

Transylvania,  propylite  of 90 

Truckeo  Range,  propylite  from 139 

Tschermak's  feldspar  theory,  evidence  in  support  of 408 

Twenty-five-hundred-foot  level,  partial  section  on  the  . .  283 
Twin  Peak,  north  (C.  4),  analysis  of  earlier  hornblende- 
andesite  from  table  following  page  151 

north,  assay  of  earlier  hornblende-andesite 

from 155 

north,  earlier  hornblende-andesite  of 34, 61, 

118,  406 

south  (C.  4),  andesite  of 87 

propylite  of  the 86,87 

Union  shaft  (D.  3),  assay  of  granular  diorite  from  the 154 

cross-section  through  the 279 

diorite  of  the 35,38,94 

eiTiptive  diorite  northeast  of  the 193 

JTtahf  diabase  in  the 280 

diorite  of  the 96,406 

earlier  diabase  in  the 198 

younger  hornblende-andesite  nearthe 34, 66, 69, 70 

Utah,  propylite  of 81 

Vein,  contents  of  the 268 

{See  Comstock  and  Lode.) 

Vein-matter,  angular  fragments  of  country  rock  in 16 

nature  of  the  so-called 271,282 

Vein-minerals,  how  dissolved 226 

origin  of 221 

precipitation  of,  from  solution 226 

Veins,  application  of  theory  of  faulting  to 186,379 

Ventilation  of  the  mines,  Church  on d 

Vertical  longitudinal  projection  of  bonanzas 284 

Virginia  and  Gold  Hill,  population  of 4 


422 


GENERAL  ESTDEX. 


Page. 

Virt!;ima  range,  King  on 25 

Vivian,  assay  of  earlier  homblende-andesite  from  near 

the 155 

V'olcanic  action  as  a  scarce  of  heat  at  Washoe 231,  238 

Volcano,  metamorpbic  diorite  near  the 34, 43 

Wagon  Canon,  propylite  from 140 

Wales  Consolidated,  granite  at  the 190 

^  metamorpbic  diorite  at  the  . .  .43, 190, 195 

Wall,  east,  impregnation  of,  von  Kichthofen  on 15 

indistinctness  of  tbe 273 

Wall  rock,  particles  of,  accompanying  ore,  von  Richt- 

hofen  on 17 

west,  relations  to  slope  of  Mount  Davidson,  von 

Ricbtbofen  on 14 

WaUs  of  tbe  Lode 267,393 

litbological  character  of,  von  Ricbtbofen  on 15 

Waller  Defeat  shaft  (D.  5),  analysis  of  diorite  from  near.. 

table  following  page  151 
assay  of  porpbyritic   diorite   from 

near 154 

Ward,  earlier  diabase  near  the 51, 197 

Washoe  District,  bullion  product  of  tbe 10, 11 

mines.    {See  Mines.) 
Water,  influence  of  the  narrowness  of  the  vein  on  the 

path  of  rising 283 

mine,  analysis  of 152 

of  tbe  mines,  possible  explanation  of  the  hea<l  of.  243 

possible  origin  in  tbe  Sierra 242 

source  of,  unexplained 241 

variations  of  temperature  of  the.  - .  242 

source  of  hot 390 

supply  of  the  Comstock 3 

the  vehicle  of  heat 265 

Weathering  of  rock  masses 371 

Werlau,  experiments  made  by  von  Strombeck  at , 310 

West  Gate,  propylite  from 142 

West  wall,  character  of,  King  on 24 

Whitney,  J.  D.,  on  the  age  of  the  metamorpbic  rocks  of 

tbe  District 13 

Wire,  faults  in   320,327,328,363 


Page. 

Woodward,  R.  W.,  analysis  by table  following  page  151 

Workings,  extent  of  the 284 

Wrmkle,  L.  F.  J.,  mapping  by 284 

Yellow  Jacket,  analysis  of  clay  from  the table  follow- 
ing page  151 

analysis  of  ore  from  the 153 

analysis  of  propylite  horse  from  tbe 

table  following  page  151 

analysis  of  water  from  the 152 

cross-section  through  the 270 

diabase  in  the 51,115, 198 

eruptive  diorite  in  the 192 

later  diabase  in  tbe 199 

slates  in  the 191 

temperatures  of  the 231,  244, 250 

water  of  the 230 

Yellow  Jacket  shaft,  equation  between  temperature  and 

depth  in  the 262 

temperature  obsei-vations  in  the 230, 

244,  250,  *260 
Yield.    (iSeeProdnct,  Gold,  Silver,  Bonanzas,  Richthofen, 
Lord.) 

of  ores,  von  Ricbtbofen  on 18 

Zacat-ecas,  propylitic  character  of 13 

Zeolites,  occurrence  of,  von  Richthofen  on 17 

Zero  method 295,296,320,353 

Zircon  in  earlier  hombleude-andesit« 56 

in  eruptive  diorite 38,39,40 

in  granite 34 

in  metamoi'phic  diorite 44 

in  quai-tz-porph>Ty 45,47 

in  slides 92, 94, 96, 98, 104, 108, 109, 119, 142 

Zirkel,  F.,  on  dacite 13 

on  diorite 12 

on  feldspar  in  later  homblende-andesite 69 

on  propylite 81 

on  propylite  and  andeait^ 27 

on  quartz-porphyry  47,111 

report  on  the  Washoe  rocks  by 26 

Zonal  structure  of  plagioclase 37,01,67 


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